diff --git a/tests/test_api.py b/tests/test_api.py
index bee464a3..c4a8f80b 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -1,9 +1,9 @@
 import json
-from typing import Dict, Optional, Tuple, List, Any
+from typing import Dict, Optional, Any
 from unittest import TestCase
 
 import optuna
-from .wsgi_utils import create_wsgi_env, send_request
+from .wsgi_utils import create_wsgi_env, send_request, WSGIEnv
 from optuna_dashboard.app import create_app
 
 
@@ -13,7 +13,7 @@ def create_json_api_wsgi_env(
     json_body: Optional[Dict[str, Any]] = None,
     content_type: str = "application/json",
     headers: Optional[Dict[str, str]] = None,
-):
+) -> WSGIEnv:
     body = json.dumps(json_body) if json_body else ""
     return create_wsgi_env(
         path, method, body, content_type=content_type, headers=headers
@@ -21,7 +21,7 @@ def create_json_api_wsgi_env(
 
 
 class APITestCase(TestCase):
-    def test_create_study(self):
+    def test_create_study(self) -> None:
         storage = optuna.storages.InMemoryStorage()
         self.assertEqual(len(storage.get_all_study_summaries()), 0)
 
@@ -38,7 +38,7 @@ class APITestCase(TestCase):
         self.assertEqual(status, "201 Created")
         self.assertEqual(len(storage.get_all_study_summaries()), 1)
 
-    def test_create_study_duplicated(self):
+    def test_create_study_duplicated(self) -> None:
         storage = optuna.storages.InMemoryStorage()
         storage.create_new_study("foo")
         self.assertEqual(len(storage.get_all_study_summaries()), 1)
@@ -56,7 +56,7 @@ class APITestCase(TestCase):
         self.assertEqual(status, "400 Bad Request")
         self.assertEqual(len(storage.get_all_study_summaries()), 1)
 
-    def test_delete_study(self):
+    def test_delete_study(self) -> None:
         storage = optuna.storages.InMemoryStorage()
         storage.create_new_study("foo1")
         storage.create_new_study("foo2")
@@ -71,7 +71,7 @@ class APITestCase(TestCase):
         self.assertEqual(status, "204 No Content")
         self.assertEqual(len(storage.get_all_study_summaries()), 1)
 
-    def test_delete_study_not_found(self):
+    def test_delete_study_not_found(self) -> None:
         storage = optuna.storages.InMemoryStorage()
         app = create_app(storage)
         env = create_json_api_wsgi_env(
diff --git a/tests/wsgi_utils.py b/tests/wsgi_utils.py
index a3a8e53f..6e911320 100644
--- a/tests/wsgi_utils.py
+++ b/tests/wsgi_utils.py
@@ -2,7 +2,7 @@ import io
 from typing import Dict, Optional, Union, Any, Tuple, List, Callable, Iterable
 
 WSGIEnv = Dict[str, Any]
-StartResponse = Callable[[bytes, List[Tuple[str, str]]], None]
+StartResponse = Callable[[str, List[Tuple[str, str]]], None]
 WSGIApp = Callable[[WSGIEnv, StartResponse], Iterable[bytes]]