diff --git a/src/router/linear-router/router.ts b/src/router/linear-router/router.ts
index 6a490448..28796ec7 100644
--- a/src/router/linear-router/router.ts
+++ b/src/router/linear-router/router.ts
@@ -10,7 +10,7 @@ const splitPathRe = /\/(:\w+(?:{(?:(?:{[\d,]+})|[^}])+})?)|\/[^\/\?]+|(\?)/g
 const splitByStarRe = /\*/
 export class LinearRouter<T> implements Router<T> {
   name: string = 'LinearRouter'
-  routes: [string, string, T][] = []
+  #routes: [string, string, T][] = []
 
   add(method: string, path: string, handler: T) {
     for (
@@ -18,14 +18,14 @@ export class LinearRouter<T> implements Router<T> {
       i < len;
       i++
     ) {
-      this.routes.push([method, paths[i], handler])
+      this.#routes.push([method, paths[i], handler])
     }
   }
 
   match(method: string, path: string): Result<T> {
     const handlers: [T, Params][] = []
-    ROUTES_LOOP: for (let i = 0, len = this.routes.length; i < len; i++) {
-      const [routeMethod, routePath, handler] = this.routes[i]
+    ROUTES_LOOP: for (let i = 0, len = this.#routes.length; i < len; i++) {
+      const [routeMethod, routePath, handler] = this.#routes[i]
       if (routeMethod === method || routeMethod === METHOD_NAME_ALL) {
         if (routePath === '*' || routePath === '/*') {
           handlers.push([handler, emptyParams])
diff --git a/src/router/reg-exp-router/node.ts b/src/router/reg-exp-router/node.ts
index c894c898..91abea7f 100644
--- a/src/router/reg-exp-router/node.ts
+++ b/src/router/reg-exp-router/node.ts
@@ -43,9 +43,9 @@ function compareKey(a: string, b: string): number {
 }
 
 export class Node {
-  index?: number
-  varIndex?: number
-  children: Record<string, Node> = Object.create(null)
+  #index?: number
+  #varIndex?: number
+  #children: Record<string, Node> = Object.create(null)
 
   insert(
     tokens: readonly string[],
@@ -55,14 +55,14 @@ export class Node {
     pathErrorCheckOnly: boolean
   ): void {
     if (tokens.length === 0) {
-      if (this.index !== undefined) {
+      if (this.#index !== undefined) {
         throw PATH_ERROR
       }
       if (pathErrorCheckOnly) {
         return
       }
 
-      this.index = index
+      this.#index = index
       return
     }
 
@@ -88,10 +88,10 @@ export class Node {
         }
       }
 
-      node = this.children[regexpStr]
+      node = this.#children[regexpStr]
       if (!node) {
         if (
-          Object.keys(this.children).some(
+          Object.keys(this.#children).some(
             (k) => k !== ONLY_WILDCARD_REG_EXP_STR && k !== TAIL_WILDCARD_REG_EXP_STR
           )
         ) {
@@ -100,19 +100,19 @@ export class Node {
         if (pathErrorCheckOnly) {
           return
         }
-        node = this.children[regexpStr] = new Node()
+        node = this.#children[regexpStr] = new Node()
         if (name !== '') {
-          node.varIndex = context.varIndex++
+          node.#varIndex = context.varIndex++
         }
       }
       if (!pathErrorCheckOnly && name !== '') {
-        paramMap.push([name, node.varIndex as number])
+        paramMap.push([name, node.#varIndex as number])
       }
     } else {
-      node = this.children[token]
+      node = this.#children[token]
       if (!node) {
         if (
-          Object.keys(this.children).some(
+          Object.keys(this.#children).some(
             (k) =>
               k.length > 1 && k !== ONLY_WILDCARD_REG_EXP_STR && k !== TAIL_WILDCARD_REG_EXP_STR
           )
@@ -122,7 +122,7 @@ export class Node {
         if (pathErrorCheckOnly) {
           return
         }
-        node = this.children[token] = new Node()
+        node = this.#children[token] = new Node()
       }
     }
 
@@ -130,21 +130,21 @@ export class Node {
   }
 
   buildRegExpStr(): string {
-    const childKeys = Object.keys(this.children).sort(compareKey)
+    const childKeys = Object.keys(this.#children).sort(compareKey)
 
     const strList = childKeys.map((k) => {
-      const c = this.children[k]
+      const c = this.#children[k]
       return (
-        (typeof c.varIndex === 'number'
-          ? `(${k})@${c.varIndex}`
+        (typeof c.#varIndex === 'number'
+          ? `(${k})@${c.#varIndex}`
           : regExpMetaChars.has(k)
           ? `\\${k}`
           : k) + c.buildRegExpStr()
       )
     })
 
-    if (typeof this.index === 'number') {
-      strList.unshift(`#${this.index}`)
+    if (typeof this.#index === 'number') {
+      strList.unshift(`#${this.#index}`)
     }
 
     if (strList.length === 0) {
diff --git a/src/router/reg-exp-router/router.ts b/src/router/reg-exp-router/router.ts
index c3358885..a9f6285c 100644
--- a/src/router/reg-exp-router/router.ts
+++ b/src/router/reg-exp-router/router.ts
@@ -123,16 +123,17 @@ function findMiddleware<T>(
 
 export class RegExpRouter<T> implements Router<T> {
   name: string = 'RegExpRouter'
-  middleware?: Record<string, Record<string, HandlerWithMetadata<T>[]>>
-  routes?: Record<string, Record<string, HandlerWithMetadata<T>[]>>
+  #middleware?: Record<string, Record<string, HandlerWithMetadata<T>[]>>
+  #routes?: Record<string, Record<string, HandlerWithMetadata<T>[]>>
 
   constructor() {
-    this.middleware = { [METHOD_NAME_ALL]: Object.create(null) }
-    this.routes = { [METHOD_NAME_ALL]: Object.create(null) }
+    this.#middleware = { [METHOD_NAME_ALL]: Object.create(null) }
+    this.#routes = { [METHOD_NAME_ALL]: Object.create(null) }
   }
 
   add(method: string, path: string, handler: T) {
-    const { middleware, routes } = this
+    const middleware = this.#middleware
+    const routes = this.#routes
 
     if (!middleware || !routes) {
       throw new Error(MESSAGE_MATCHER_IS_ALREADY_BUILT)
@@ -232,14 +233,14 @@ export class RegExpRouter<T> implements Router<T> {
   #buildAllMatchers(): Record<string, Matcher<T> | null> {
     const matchers: Record<string, Matcher<T> | null> = Object.create(null)
 
-    Object.keys(this.routes!)
-      .concat(Object.keys(this.middleware!))
+    Object.keys(this.#routes!)
+      .concat(Object.keys(this.#middleware!))
       .forEach((method) => {
         matchers[method] ||= this.#buildMatcher(method)
       })
 
     // Release cache
-    this.middleware = this.routes = undefined
+    this.#middleware = this.#routes = undefined
 
     return matchers
   }
@@ -249,7 +250,7 @@ export class RegExpRouter<T> implements Router<T> {
 
     let hasOwnRoute = method === METHOD_NAME_ALL
 
-    ;[this.middleware!, this.routes!].forEach((r) => {
+    ;[this.#middleware!, this.#routes!].forEach((r) => {
       const ownRoute = r[method]
         ? Object.keys(r[method]).map((path) => [path, r[method][path]])
         : []
diff --git a/src/router/reg-exp-router/trie.ts b/src/router/reg-exp-router/trie.ts
index 39bf99f7..f7daacad 100644
--- a/src/router/reg-exp-router/trie.ts
+++ b/src/router/reg-exp-router/trie.ts
@@ -4,8 +4,8 @@ import { Node } from './node'
 export type ReplacementMap = number[]
 
 export class Trie {
-  context: Context = { varIndex: 0 }
-  root: Node = new Node()
+  #context: Context = { varIndex: 0 }
+  #root: Node = new Node()
 
   insert(path: string, index: number, pathErrorCheckOnly: boolean): ParamAssocArray {
     const paramAssoc: ParamAssocArray = []
@@ -41,13 +41,13 @@ export class Trie {
       }
     }
 
-    this.root.insert(tokens, index, paramAssoc, this.context, pathErrorCheckOnly)
+    this.#root.insert(tokens, index, paramAssoc, this.#context, pathErrorCheckOnly)
 
     return paramAssoc
   }
 
   buildRegExp(): [RegExp, ReplacementMap, ReplacementMap] {
-    let regexp = this.root.buildRegExpStr()
+    let regexp = this.#root.buildRegExpStr()
     if (regexp === '') {
       return [/^$/, [], []] // never match
     }
diff --git a/src/router/smart-router/router.ts b/src/router/smart-router/router.ts
index 8e7a11a5..9ec464da 100644
--- a/src/router/smart-router/router.ts
+++ b/src/router/smart-router/router.ts
@@ -3,27 +3,29 @@ import { MESSAGE_MATCHER_IS_ALREADY_BUILT, UnsupportedPathError } from '../../ro
 
 export class SmartRouter<T> implements Router<T> {
   name: string = 'SmartRouter'
-  routers: Router<T>[] = []
-  routes?: [string, string, T][] = []
+  #routers: Router<T>[] = []
+  #routes?: [string, string, T][] = []
 
-  constructor(init: Pick<SmartRouter<T>, 'routers'>) {
-    Object.assign(this, init)
+  constructor(init: { routers: Router<T>[] }) {
+    this.#routers = init.routers
   }
 
   add(method: string, path: string, handler: T) {
-    if (!this.routes) {
+    if (!this.#routes) {
       throw new Error(MESSAGE_MATCHER_IS_ALREADY_BUILT)
     }
 
-    this.routes.push([method, path, handler])
+    this.#routes.push([method, path, handler])
   }
 
   match(method: string, path: string): Result<T> {
-    if (!this.routes) {
+    if (!this.#routes) {
       throw new Error('Fatal error')
     }
 
-    const { routers, routes } = this
+    const routers = this.#routers
+    const routes = this.#routes
+
     const len = routers.length
     let i = 0
     let res
@@ -42,8 +44,8 @@ export class SmartRouter<T> implements Router<T> {
       }
 
       this.match = router.match.bind(router)
-      this.routers = [router]
-      this.routes = undefined
+      this.#routers = [router]
+      this.#routes = undefined
       break
     }
 
@@ -59,10 +61,10 @@ export class SmartRouter<T> implements Router<T> {
   }
 
   get activeRouter(): Router<T> {
-    if (this.routes || this.routers.length !== 1) {
+    if (this.#routes || this.#routers.length !== 1) {
       throw new Error('No active router has been determined yet.')
     }
 
-    return this.routers[0]
+    return this.#routers[0]
   }
 }
diff --git a/src/router/trie-router/node.ts b/src/router/trie-router/node.ts
index 2d0ae6c3..1efe89d2 100644
--- a/src/router/trie-router/node.ts
+++ b/src/router/trie-router/node.ts
@@ -14,26 +14,26 @@ type HandlerParamsSet<T> = HandlerSet<T> & {
 }
 
 export class Node<T> {
-  methods: Record<string, HandlerSet<T>>[]
+  #methods: Record<string, HandlerSet<T>>[]
 
-  children: Record<string, Node<T>>
-  patterns: Pattern[]
-  order: number = 0
-  params: Record<string, string> = Object.create(null)
+  #children: Record<string, Node<T>>
+  #patterns: Pattern[]
+  #order: number = 0
+  #params: Record<string, string> = Object.create(null)
 
   constructor(method?: string, handler?: T, children?: Record<string, Node<T>>) {
-    this.children = children || Object.create(null)
-    this.methods = []
+    this.#children = children || Object.create(null)
+    this.#methods = []
     if (method && handler) {
       const m: Record<string, HandlerSet<T>> = Object.create(null)
       m[method] = { handler, possibleKeys: [], score: 0 }
-      this.methods = [m]
+      this.#methods = [m]
     }
-    this.patterns = []
+    this.#patterns = []
   }
 
   insert(method: string, path: string, handler: T): Node<T> {
-    this.order = ++this.order
+    this.#order = ++this.#order
 
     // eslint-disable-next-line @typescript-eslint/no-this-alias
     let curNode: Node<T> = this
@@ -44,8 +44,8 @@ export class Node<T> {
     for (let i = 0, len = parts.length; i < len; i++) {
       const p: string = parts[i]
 
-      if (Object.keys(curNode.children).includes(p)) {
-        curNode = curNode.children[p]
+      if (Object.keys(curNode.#children).includes(p)) {
+        curNode = curNode.#children[p]
         const pattern = getPattern(p)
         if (pattern) {
           possibleKeys.push(pattern[1])
@@ -53,14 +53,14 @@ export class Node<T> {
         continue
       }
 
-      curNode.children[p] = new Node()
+      curNode.#children[p] = new Node()
 
       const pattern = getPattern(p)
       if (pattern) {
-        curNode.patterns.push(pattern)
+        curNode.#patterns.push(pattern)
         possibleKeys.push(pattern[1])
       }
-      curNode = curNode.children[p]
+      curNode = curNode.#children[p]
     }
 
     const m: Record<string, HandlerSet<T>> = Object.create(null)
@@ -68,11 +68,11 @@ export class Node<T> {
     const handlerSet: HandlerSet<T> = {
       handler,
       possibleKeys: possibleKeys.filter((v, i, a) => a.indexOf(v) === i),
-      score: this.order,
+      score: this.#order,
     }
 
     m[method] = handlerSet
-    curNode.methods.push(m)
+    curNode.#methods.push(m)
 
     return curNode
   }
@@ -85,8 +85,8 @@ export class Node<T> {
     params: Record<string, string>
   ): HandlerParamsSet<T>[] {
     const handlerSets: HandlerParamsSet<T>[] = []
-    for (let i = 0, len = node.methods.length; i < len; i++) {
-      const m = node.methods[i]
+    for (let i = 0, len = node.#methods.length; i < len; i++) {
+      const m = node.#methods[i]
       const handlerSet = (m[method] || m[METHOD_NAME_ALL]) as HandlerParamsSet<T>
       const processedSet: Record<number, boolean> = {}
       if (handlerSet !== undefined) {
@@ -107,7 +107,7 @@ export class Node<T> {
 
   search(method: string, path: string): [[T, Params][]] {
     const handlerSets: HandlerParamsSet<T>[] = []
-    this.params = Object.create(null)
+    this.#params = Object.create(null)
 
     // eslint-disable-next-line @typescript-eslint/no-this-alias
     const curNode: Node<T> = this
@@ -121,42 +121,42 @@ export class Node<T> {
 
       for (let j = 0, len2 = curNodes.length; j < len2; j++) {
         const node = curNodes[j]
-        const nextNode = node.children[part]
+        const nextNode = node.#children[part]
 
         if (nextNode) {
-          nextNode.params = node.params
+          nextNode.#params = node.#params
           if (isLast) {
             // '/hello/*' => match '/hello'
-            if (nextNode.children['*']) {
+            if (nextNode.#children['*']) {
               handlerSets.push(
                 ...this.#getHandlerSets(
-                  nextNode.children['*'],
+                  nextNode.#children['*'],
                   method,
-                  node.params,
+                  node.#params,
                   Object.create(null)
                 )
               )
             }
             handlerSets.push(
-              ...this.#getHandlerSets(nextNode, method, node.params, Object.create(null))
+              ...this.#getHandlerSets(nextNode, method, node.#params, Object.create(null))
             )
           } else {
             tempNodes.push(nextNode)
           }
         }
 
-        for (let k = 0, len3 = node.patterns.length; k < len3; k++) {
-          const pattern = node.patterns[k]
+        for (let k = 0, len3 = node.#patterns.length; k < len3; k++) {
+          const pattern = node.#patterns[k]
 
-          const params = { ...node.params }
+          const params = { ...node.#params }
 
           // Wildcard
           // '/hello/*/foo' => match /hello/bar/foo
           if (pattern === '*') {
-            const astNode = node.children['*']
+            const astNode = node.#children['*']
             if (astNode) {
               handlerSets.push(
-                ...this.#getHandlerSets(astNode, method, node.params, Object.create(null))
+                ...this.#getHandlerSets(astNode, method, node.#params, Object.create(null))
               )
               tempNodes.push(astNode)
             }
@@ -169,27 +169,27 @@ export class Node<T> {
 
           const [key, name, matcher] = pattern
 
-          const child = node.children[key]
+          const child = node.#children[key]
 
           // `/js/:filename{[a-z]+.js}` => match /js/chunk/123.js
           const restPathString = parts.slice(i).join('/')
           if (matcher instanceof RegExp && matcher.test(restPathString)) {
             params[name] = restPathString
-            handlerSets.push(...this.#getHandlerSets(child, method, node.params, params))
+            handlerSets.push(...this.#getHandlerSets(child, method, node.#params, params))
             continue
           }
 
           if (matcher === true || matcher.test(part)) {
             params[name] = part
             if (isLast) {
-              handlerSets.push(...this.#getHandlerSets(child, method, params, node.params))
-              if (child.children['*']) {
+              handlerSets.push(...this.#getHandlerSets(child, method, params, node.#params))
+              if (child.#children['*']) {
                 handlerSets.push(
-                  ...this.#getHandlerSets(child.children['*'], method, params, node.params)
+                  ...this.#getHandlerSets(child.#children['*'], method, params, node.#params)
                 )
               }
             } else {
-              child.params = params
+              child.#params = params
               tempNodes.push(child)
             }
           }
diff --git a/src/router/trie-router/router.ts b/src/router/trie-router/router.ts
index 65700ffd..65b99788 100644
--- a/src/router/trie-router/router.ts
+++ b/src/router/trie-router/router.ts
@@ -4,25 +4,25 @@ import { Node } from './node'
 
 export class TrieRouter<T> implements Router<T> {
   name: string = 'TrieRouter'
-  node: Node<T>
+  #node: Node<T>
 
   constructor() {
-    this.node = new Node()
+    this.#node = new Node()
   }
 
   add(method: string, path: string, handler: T) {
     const results = checkOptionalParameter(path)
     if (results) {
       for (let i = 0, len = results.length; i < len; i++) {
-        this.node.insert(method, results[i], handler)
+        this.#node.insert(method, results[i], handler)
       }
       return
     }
 
-    this.node.insert(method, path, handler)
+    this.#node.insert(method, path, handler)
   }
 
   match(method: string, path: string): Result<T> {
-    return this.node.search(method, path)
+    return this.#node.search(method, path)
   }
 }