diff --git a/optuna/distributions.py b/optuna/distributions.py
index 9642efd835..fd5659c13e 100644
--- a/optuna/distributions.py
+++ b/optuna/distributions.py
@@ -421,7 +421,7 @@ def __init__(self, low: int, high: int, step: int = 1) -> None:
         super().__init__(low=low, high=high, log=False, step=step)
 
     def _asdict(self) -> dict:
-        d = copy.deepcopy(self.__dict__)
+        d = self.__dict__.copy()
         d.pop("log")
         return d