diff --git a/.eslintignore b/.eslintignore
new file mode 100644
index 00000000..544edd99
--- /dev/null
+++ b/.eslintignore
@@ -0,0 +1 @@
+.eslintrc.cjs
\ No newline at end of file
diff --git a/.eslintrc.cjs b/.eslintrc.cjs
index 1a72e8c6..8c7b1356 100644
--- a/.eslintrc.cjs
+++ b/.eslintrc.cjs
@@ -1,3 +1,9 @@
 module.exports = {
   extends: ['@hono/eslint-config'],
+  rules: {
+    '@typescript-eslint/unbound-method': 'error',
+  },
+  parserOptions: {
+    project: ['./tsconfig.json', './runtime_tests/tsconfig.json'],
+  },
 }
diff --git a/deno_dist/helper/css/index.ts b/deno_dist/helper/css/index.ts
index 555638d6..9b737e77 100644
--- a/deno_dist/helper/css/index.ts
+++ b/deno_dist/helper/css/index.ts
@@ -101,6 +101,7 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
     ;(className as HtmlEscapedString).callbacks = [addClassNameToContext]
     const promise = Promise.resolve(className)
     Object.assign(promise, cssClassName)
+    // eslint-disable-next-line @typescript-eslint/unbound-method
     promise.toString = cssJsxDomObject.toString
     return promise
   }
diff --git a/deno_dist/jsx/dom/css.ts b/deno_dist/jsx/dom/css.ts
index 913fb0df..29917d72 100644
--- a/deno_dist/jsx/dom/css.ts
+++ b/deno_dist/jsx/dom/css.ts
@@ -133,6 +133,7 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
   const [cssObject, Style] = createCssJsxDomObjects({ id })
 
   const newCssClassNameObject = (cssClassName: CssClassName): string => {
+    // eslint-disable-next-line @typescript-eslint/unbound-method
     cssClassName.toString = cssObject.toString
     return cssClassName as unknown as string
   }
diff --git a/runtime_tests/tsconfig.json b/runtime_tests/tsconfig.json
new file mode 100644
index 00000000..6a9a5d7f
--- /dev/null
+++ b/runtime_tests/tsconfig.json
@@ -0,0 +1,7 @@
+{
+  "extends": "../tsconfig.json",
+  "include": [
+    "**/*.ts",
+    "**/*.tsx"
+  ]
+}
\ No newline at end of file
diff --git a/src/helper/css/index.ts b/src/helper/css/index.ts
index cbb00986..a7734683 100644
--- a/src/helper/css/index.ts
+++ b/src/helper/css/index.ts
@@ -101,6 +101,7 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
     ;(className as HtmlEscapedString).callbacks = [addClassNameToContext]
     const promise = Promise.resolve(className)
     Object.assign(promise, cssClassName)
+    // eslint-disable-next-line @typescript-eslint/unbound-method
     promise.toString = cssJsxDomObject.toString
     return promise
   }
diff --git a/src/helper/ssg/ssg.test.tsx b/src/helper/ssg/ssg.test.tsx
index 31510360..6f8bb819 100644
--- a/src/helper/ssg/ssg.test.tsx
+++ b/src/helper/ssg/ssg.test.tsx
@@ -1,3 +1,4 @@
+/* eslint-disable @typescript-eslint/unbound-method */
 import { beforeEach, describe, expect, it } from 'vitest'
 import { Hono } from '../../hono'
 // eslint-disable-next-line @typescript-eslint/no-unused-vars
diff --git a/src/jsx/dom/css.ts b/src/jsx/dom/css.ts
index 6edcabb8..4629ef9c 100644
--- a/src/jsx/dom/css.ts
+++ b/src/jsx/dom/css.ts
@@ -133,6 +133,7 @@ export const createCssContext = ({ id }: { id: Readonly<string> }) => {
   const [cssObject, Style] = createCssJsxDomObjects({ id })
 
   const newCssClassNameObject = (cssClassName: CssClassName): string => {
+    // eslint-disable-next-line @typescript-eslint/unbound-method
     cssClassName.toString = cssObject.toString
     return cssClassName as unknown as string
   }