diff --git a/optuna_dashboard/_serializer.py b/optuna_dashboard/_serializer.py
index 5ac9c55b..35b9d6a7 100644
--- a/optuna_dashboard/_serializer.py
+++ b/optuna_dashboard/_serializer.py
@@ -25,9 +25,6 @@ if TYPE_CHECKING:
     from typing import Literal
     from typing import TypedDict
 
-    from ._preferential_history import ChooseWorstHistory
-    from ._preferential_history import History
-
     Attribute = TypedDict(
         "Attribute",
         {