diff --git a/package/samplers/hill_climb_search/hill_climb_search.py b/package/samplers/hill_climb_search/hill_climb_search.py
index 24e946ed..d2c671af 100644
--- a/package/samplers/hill_climb_search/hill_climb_search.py
+++ b/package/samplers/hill_climb_search/hill_climb_search.py
@@ -129,7 +129,7 @@ def sample_relative(
 
                 criteria = (
                     neighbor_value < self._current_point_value
-                    if study.direction == "minimize"
+                    if study.direction == optuna.study.StudyDirection.MINIMIZE
                     else neighbor_value > self._current_point_value
                 )