diff --git a/optuna_dashboard/_app.py b/optuna_dashboard/_app.py
index 2ad7874e..d7ddf72b 100644
--- a/optuna_dashboard/_app.py
+++ b/optuna_dashboard/_app.py
@@ -316,7 +316,7 @@ def create_app(
         except ValueError:
             response.status = 400
             return {"reason": "invalid request."}
-        if component_type not in ["Note", "Artifact"]:
+        if component_type not in ["note", "artifact"]:
             response.status = 400
             return {"reason": "component_type must be either 'Note' or 'Artifact'."}
 
diff --git a/optuna_dashboard/_preference_setting.py b/optuna_dashboard/_preference_setting.py
index d9d7c1c2..2310aa03 100644
--- a/optuna_dashboard/_preference_setting.py
+++ b/optuna_dashboard/_preference_setting.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 from typing import TYPE_CHECKING
+from typing import Any
 
 from optuna.storages import BaseStorage
 
@@ -10,7 +11,7 @@ from .preferential._study import PreferentialStudy
 if TYPE_CHECKING:
     from typing import Literal
 
-    OUTPUT_COMPONENT_TYPE = Literal["Note", "Artifact"]
+    OUTPUT_COMPONENT_TYPE = Literal["note", "artifact"]
 
 _SYSTEM_ATTR_FEEDBACK_COMPONENT = "preference:component"
 
@@ -19,22 +20,22 @@ def _register_preference_feedback_component_type(
     study_id: int,
     storage: BaseStorage,
     component_type: OUTPUT_COMPONENT_TYPE,
-    artifact_key: str = "",
+    artifact_key: str | None = None,
 ) -> None:
+    value: dict[str, Any] = {"type": component_type}
+    if artifact_key is not None:
+        value["artifact_key"] = artifact_key
     storage.set_study_system_attr(
         study_id=study_id,
         key=_SYSTEM_ATTR_FEEDBACK_COMPONENT,
-        value={
-            "type": component_type,
-            "artifact_key": artifact_key,
-        },
+        value=value,
     )
 
 
 def register_preference_feedback_component_type(
     study: PreferentialStudy,
     component_type: OUTPUT_COMPONENT_TYPE,
-    artifact_key: str = "",
+    artifact_key: str | None = None,
 ) -> None:
     """Register output component to the study.
 
@@ -48,6 +49,11 @@ def register_preference_feedback_component_type(
             this argument is used as the attribute key of the artifact.
             Each trial displays the artifact whose id is the value of the attribute.
     """
+    if component_type == "artifact":
+        assert (
+            artifact_key is not None
+        ), "artifact_key must be specified when component_type is Artifact"
+
     _register_preference_feedback_component_type(
         study_id=study._study._study_id,
         storage=study._study._storage,
diff --git a/python_tests/test_api.py b/python_tests/test_api.py
index 1908c188..c478dc26 100644
--- a/python_tests/test_api.py
+++ b/python_tests/test_api.py
@@ -183,7 +183,7 @@ class APITestCase(TestCase):
     def test_change_component(self) -> None:
         storage = optuna.storages.InMemoryStorage()
         study = create_study(storage=storage, n_generate=3)
-        register_preference_feedback_component_type(study, "Note")
+        register_preference_feedback_component_type(study, "note")
         for _ in range(3):
             study.ask()
 
@@ -193,7 +193,7 @@ class APITestCase(TestCase):
             app,
             f"/api/studies/{study_id}/preference_feedback_component_type",
             "PUT",
-            body=json.dumps({"type": "Artifact", "artifact_key": "image"}),
+            body=json.dumps({"type": "artifact", "artifact_key": "image"}),
             content_type="application/json",
         )
         self.assertEqual(status, 204)
@@ -207,7 +207,7 @@ class APITestCase(TestCase):
         self.assertEqual(status, 200)
 
         study_detail = json.loads(body)
-        assert study_detail["feedback_component_type"]["type"] == "Artifact"
+        assert study_detail["feedback_component_type"]["type"] == "artifact"
         assert study_detail["feedback_component_type"]["artifact_key"] == "image"
 
     def test_skip_trial(self) -> None:
diff --git a/python_tests/test_preference_setting.py b/python_tests/test_preference_setting.py
index 8a601af7..bcd53e42 100644
--- a/python_tests/test_preference_setting.py
+++ b/python_tests/test_preference_setting.py
@@ -11,10 +11,10 @@ from optuna_dashboard.preferential._study import PreferentialStudy
 class FeedbackSettingTestCase(TestCase):
     def test_widget_to_dict_from_dict(self) -> None:
         study = PreferentialStudy(optuna.create_study())
-        register_preference_feedback_component_type(study, "Artifact", "image_key")
+        register_preference_feedback_component_type(study, "artifact", "image_key")
         system_attrs = study._study.system_attrs
         feedback_type = system_attrs.get(_SYSTEM_ATTR_FEEDBACK_COMPONENT, {})
         assert "type" in feedback_type
-        assert feedback_type["type"] == "Artifact"
+        assert feedback_type["type"] == "artifact"
         assert "artifact_key" in feedback_type
         assert feedback_type["artifact_key"] == "image_key"