diff --git a/ch06/hyperparameter_optimization.py b/ch06/hyperparameter_optimization.py
index 0d8004b0..f31b0e05 100644
--- a/ch06/hyperparameter_optimization.py
+++ b/ch06/hyperparameter_optimization.py
@@ -16,7 +16,7 @@
 
 # 20%를 검증 데이터로 분할
 validation_rate = 0.20
-validation_num = x_train.shape[0] * validation_rate
+validation_num = int(x_train.shape[0] * validation_rate)
 x_train, t_train = shuffle_dataset(x_train, t_train)
 x_val = x_train[:validation_num]
 t_val = t_train[:validation_num]