diff --git a/tests/test_api.py b/tests/test_api.py
index 1e545274..3226ecc5 100644
--- a/tests/test_api.py
+++ b/tests/test_api.py
@@ -24,9 +24,10 @@ class APITestCase(TestCase):
         self.assertEqual(len(study_summaries), 2)
 
     def test_create_study(self) -> None:
-        for name, directions in [
-            ("single-objective", ["minimize"]),
-            ("multi-objective", ["minimize", "maximize"]),
+        for name, directions, expected_status in [
+            ("single-objective success", ["minimize"], 201),
+            ("multi-objective success", ["minimize", "maximize"], 201),
+            ("invalid direction name", ["invalid-direction", "maximize"], 400),
         ]:
             with self.subTest(name):
                 storage = optuna.storages.InMemoryStorage()
@@ -44,8 +45,12 @@ class APITestCase(TestCase):
                     content_type="application/json",
                     body=json.dumps(request_body),
                 )
-                self.assertEqual(status, 201)
-                self.assertEqual(len(storage.get_all_study_summaries()), 1)
+                self.assertEqual(status, expected_status)
+
+                if expected_status == 201:
+                    self.assertEqual(len(storage.get_all_study_summaries()), 1)
+                else:
+                    self.assertEqual(len(storage.get_all_study_summaries()), 0)
 
     def test_create_study_duplicated(self) -> None:
         storage = optuna.storages.InMemoryStorage()