diff --git a/src/listener.ts b/src/listener.ts
index 504c528..6a9f207 100644
--- a/src/listener.ts
+++ b/src/listener.ts
@@ -45,6 +45,14 @@ const handleResponseError = (e: unknown, outgoing: ServerResponse | Http2ServerR
   }
 }
 
+const flushHeaders = (outgoing: ServerResponse | Http2ServerResponse) => {
+  // If outgoing is ServerResponse (HTTP/1.1), it requires this to flush headers.
+  // However, Http2ServerResponse is sent without this.
+  if ('flushHeaders' in outgoing && outgoing.writable) {
+    outgoing.flushHeaders()
+  }
+}
+
 const responseViaCache = async (
   res: Response,
   outgoing: ServerResponse | Http2ServerResponse
@@ -69,6 +77,7 @@ const responseViaCache = async (
   } else if (body instanceof Blob) {
     outgoing.end(new Uint8Array(await body.arrayBuffer()))
   } else {
+    flushHeaders(outgoing)
     return writeFromReadableStream(body, outgoing)?.catch(
       (e) => handleResponseError(e, outgoing) as undefined
     )
@@ -129,6 +138,7 @@ const responseViaResponseObject = async (
       !regContentType.test(contentType as string)
     ) {
       outgoing.writeHead(res.status, resHeaderRecord)
+      flushHeaders(outgoing)
 
       await writeFromReadableStream(res.body, outgoing)
     } else {
diff --git a/test/server.test.ts b/test/server.test.ts
index 745efc9..1d524f5 100644
--- a/test/server.test.ts
+++ b/test/server.test.ts
@@ -134,9 +134,14 @@ describe('various response body types', () => {
     })
     return response
   })
+  let resolveReadableStreamPromise: () => void
+  const readableStreamPromise = new Promise<void>((resolve) => {
+    resolveReadableStreamPromise = resolve
+  })
   app.get('/readable-stream', () => {
     const stream = new ReadableStream({
       async start(controller) {
+        await readableStreamPromise
         controller.enqueue('Hello!')
         controller.enqueue(' Node!')
         controller.close()
@@ -187,15 +192,21 @@ describe('various response body types', () => {
 
   it('Should return 200 response - GET /readable-stream', async () => {
     const expectedChunks = ['Hello!', ' Node!']
-    const res = await request(server)
+    const resPromise = request(server)
       .get('/readable-stream')
       .parse((res, fn) => {
+        // response header should be sent before sending data.
+        expect(res.headers['transfer-encoding']).toBe('chunked')
+        resolveReadableStreamPromise()
+
         res.on('data', (chunk) => {
           const str = chunk.toString()
           expect(str).toBe(expectedChunks.shift())
         })
         res.on('end', () => fn(null, ''))
       })
+    await new Promise((resolve) => setTimeout(resolve, 100))
+    const res = await resPromise
     expect(res.status).toBe(200)
     expect(res.headers['content-type']).toMatch('text/plain; charset=UTF-8')
     expect(res.headers['content-length']).toBeUndefined()