diff --git a/src/request.test.ts b/src/request.test.ts
index 61654c130..11f85fb30 100644
--- a/src/request.test.ts
+++ b/src/request.test.ts
@@ -161,6 +161,20 @@ describe('headers', () => {
     const foo = req.header('foo')
     expect(foo).toEqual('')
   })
+
+  test('Keys of the arguments for req.header() are not case-sensitive', () => {
+    const req = new HonoRequest(
+      new Request('http://localhost', {
+        headers: {
+          'Content-Type': 'application/json',
+          apikey: 'abc',
+          lowercase: 'lowercase value',
+        },
+      })
+    )
+    expect(req.header('Content-Type')).toBe('application/json')
+    expect(req.header('ApiKey')).toBe('abc')
+  })
 })
 
 const text = '{"foo":"bar"}'
diff --git a/src/request.ts b/src/request.ts
index 68c710c35..647129ab7 100644
--- a/src/request.ts
+++ b/src/request.ts
@@ -178,7 +178,7 @@ export class HonoRequest<P extends string = '/', I extends Input['out'] = {}> {
   header(): Record<RequestHeader | (string & CustomHeader), string>
   header(name?: string) {
     if (name) {
-      return this.raw.headers.get(name.toLowerCase()) ?? undefined
+      return this.raw.headers.get(name) ?? undefined
     }
 
     const headerData: Record<string, string | undefined> = {}