diff --git a/optuna_dashboard/_cached_extra_study_property.py b/optuna_dashboard/_cached_extra_study_property.py
index 1e4fb1a7..919853f4 100644
--- a/optuna_dashboard/_cached_extra_study_property.py
+++ b/optuna_dashboard/_cached_extra_study_property.py
@@ -1,6 +1,7 @@
 from __future__ import annotations
 
 import copy
+import numbers
 import threading
 from typing import Any
 from typing import List
@@ -9,7 +10,6 @@ from typing import Set
 from typing import Tuple
 from typing import TYPE_CHECKING
 
-import numpy as np
 from optuna.distributions import BaseDistribution
 from optuna.trial import FrozenTrial
 from optuna.trial import TrialState
@@ -86,11 +86,11 @@ class _CachedExtraStudyProperty:
 
         self._cursor = next_cursor
 
-    def _is_sortable_value(self, v: Any) -> bool:
-        return not isinstance(v, bool) and isinstance(v, (int, float, np.integer, np.floating))
-
     def _update_user_attrs(self, trial: FrozenTrial) -> None:
-        current_user_attrs = {k: self._is_sortable_value(v) for k, v in trial.user_attrs.items()}
+        current_user_attrs = {
+            k: not isinstance(v, bool) and isinstance(v, numbers.Real)
+            for k, v in trial.user_attrs.items()
+        }
         for attr_name, current_is_sortable in current_user_attrs.items():
             is_sortable = self._union_user_attrs.get(attr_name)
             if is_sortable is None:
diff --git a/optuna_dashboard/_serializer.py b/optuna_dashboard/_serializer.py
index 2730c328..dbcdc991 100644
--- a/optuna_dashboard/_serializer.py
+++ b/optuna_dashboard/_serializer.py
@@ -2,11 +2,11 @@ from __future__ import annotations
 
 from datetime import datetime
 import json
+import numbers
 from typing import Any
 from typing import TYPE_CHECKING
 from typing import Union
 
-import numpy as np
 from optuna.distributions import BaseDistribution
 from optuna.distributions import CategoricalDistribution
 from optuna.study import StudySummary
@@ -104,8 +104,8 @@ def serialize_attrs(attrs: dict[str, Any]) -> list[Attribute]:
             value = "<binary object>"
         elif isinstance(v, str):
             value = v
-        elif isinstance(v, (np.floating, np.integer)):
-            value = str(v.item())
+        elif isinstance(v, numbers.Real):
+            value = str(v)
         else:
             value = json.dumps(v)
             value = value[:MAX_ATTR_LENGTH] if len(value) > MAX_ATTR_LENGTH else value