diff --git a/mohou/trainer.py b/mohou/trainer.py
index d4449bf..3fb0e4e 100644
--- a/mohou/trainer.py
+++ b/mohou/trainer.py
@@ -455,6 +455,7 @@ def train(
     config: TrainConfig = TrainConfig(),
     device: Optional[torch.device] = None,
     is_stoppable: Optional[Callable[[TrainCache], bool]] = None,
+    num_workers: int = 2,
 ) -> None:
     r"""
     higher-level train function that auto create dataloader from the dataset
@@ -462,9 +463,26 @@ def train(
 
     dataset_train, dataset_validate = split_with_ratio(dataset, config.valid_data_ratio)
 
-    train_loader = DataLoader(dataset=dataset_train, batch_size=config.batch_size, shuffle=True)
+    drop_last = True
+    if len(dataset_train) < config.batch_size:
+        message = "dataset size is smaller than batch_size. drop_last is set to False"
+        logger.warn(change_color_to_yellow(message))
+        drop_last = False
+
+    # drop last is necessary for batch normalization
+    train_loader = DataLoader(
+        dataset=dataset_train,
+        batch_size=config.batch_size,
+        shuffle=True,
+        num_workers=num_workers,
+        drop_last=drop_last,
+    )
     validate_loader = DataLoader(
-        dataset=dataset_validate, batch_size=config.batch_size, shuffle=True
+        dataset=dataset_validate,
+        batch_size=config.batch_size,
+        shuffle=True,
+        num_workers=num_workers,
+        drop_last=False,  # drop_last is not necessary for validation as batch normalization is not used
     )
     train_lower(
         project_path,