diff --git a/nyaggle/experiment/gbdt.py b/nyaggle/experiment/gbdt.py
index 864ae9c..829c28b 100644
--- a/nyaggle/experiment/gbdt.py
+++ b/nyaggle/experiment/gbdt.py
@@ -3,7 +3,7 @@ import os
 import time
 from collections import namedtuple
 from datetime import datetime
-from typing import Any, Callable, Dict, Iterable, List, Optional, Union
+from typing import Any, Callable, Dict, Iterable, List, Optional, Tuple, Union
 
 import numpy as np
 import optuna.integration.lightgbm as optuna_lgb
@@ -12,6 +12,7 @@ import sklearn.utils.multiclass as multiclass
 from catboost import CatBoost, CatBoostClassifier, CatBoostRegressor
 from lightgbm import LGBMModel, LGBMClassifier, LGBMRegressor
 from more_itertools import first_true
+from pandas.api.types import is_integer_dtype, is_categorical
 from sklearn.model_selection import BaseCrossValidator
 from sklearn.metrics import roc_auc_score, mean_squared_error, log_loss
 
@@ -119,6 +120,7 @@ def experiment_gbdt(model_params: Dict[str, Any],
                     submission_filename: Optional[str] = None,
                     type_of_target: str = 'auto',
                     tuning_time_budget: Optional[int] = None,
+                    with_auto_prep: bool = True,
                     with_mlflow: bool = False,
                     mlflow_experiment_id: Optional[Union[int, str]] = None,
                     mlflow_run_name: Optional[str] = None,
@@ -196,6 +198,9 @@ def experiment_gbdt(model_params: Dict[str, Any],
         tuning_time_budget:
             If not ``None``, model parameters will be automatically updated using optuna with the specified time
              budgets in seconds (only available in lightgbm).
+        with_auto_prep:
+            If True, the input datasets will be copied and automatic preprocessing will be performed on them.
+            For example, if ``gbdt_type = 'cat'``, all missing values in categorical features will be filled.
         with_mlflow:
             If True, `mlflow tracking <https://www.mlflow.org/docs/latest/tracking.html>`_ is used.
             One instance of ``nyaggle.experiment.Experiment`` corresponds to one run in mlflow.
@@ -231,6 +236,8 @@ def experiment_gbdt(model_params: Dict[str, Any],
     cv = check_cv(cv, y)
 
     _check_input(X_train, y, X_test)
+    if with_auto_prep:
+        X_train, X_test = autoprep(X_train, X_test, categorical_feature, gbdt_type)
 
     logging_directory = logging_directory.format(time=datetime.now().strftime('%Y%m%d_%H%M%S'))
 
@@ -366,3 +373,34 @@ def _check_input(X_train: pd.DataFrame, y: pd.Series,
 
     if X_test is not None:
         assert list(X_train.columns) == list(X_test.columns), "columns are different between X_train and X_test"
+
+
+def autoprep(X_train: pd.DataFrame, X_test: Optional[pd.DataFrame],
+             categorical_feature: Optional[List[str]] = None,
+             gbdt_type: str = 'lgbm') -> Tuple[pd.DataFrame, pd.DataFrame]:
+    if categorical_feature is None:
+        categorical_feature = [c for c in X_train.columns if X_train[c].dtype.name in ['object', 'category']]
+
+    if gbdt_type == 'cat' and len(categorical_feature) > 0:
+        X_train = X_train.copy()
+        X_all = X_train if X_test is None else pd.concat([X_train, X_test])
+
+        # https://catboost.ai/docs/concepts/faq.html#why-float-and-nan-values-are-forbidden-for-cat-features
+        for c in categorical_feature:
+            if is_integer_dtype(X_train[c].dtype):
+                fillval = X_all[c].min() - 1
+            else:
+                unique_values = X_all[c].unique()
+                fillval = 'na'
+                while fillval in unique_values:
+                    fillval += '-'
+            if is_categorical(X_train[c]):
+                X_train[c] = X_train[c].cat.add_categories(fillval).fillna(fillval)
+                if X_test is not None:
+                    X_test[c] = X_test[c].cat.add_categories(fillval).fillna(fillval)
+            else:
+                X_train[c].fillna(fillval, inplace=True)
+                if X_test is not None:
+                    X_test[c].fillna(fillval, inplace=True)
+
+    return X_train, X_test
\ No newline at end of file
diff --git a/nyaggle/testing/util.py b/nyaggle/testing/util.py
index f20d91e..c5d00b3 100644
--- a/nyaggle/testing/util.py
+++ b/nyaggle/testing/util.py
@@ -31,7 +31,7 @@ def make_classification_df(n_samples: int = 1024,
 
     for i in range(n_cat_features):
         X['cat_{}'.format(i)] = \
-            pd.Series(np.random.choice(['A', 'B', None], size=n_samples)).astype(str).astype('category')
+            pd.Series(np.random.choice(['A', 'B', None], size=n_samples)).astype('category')
 
     return X, y
 
diff --git a/tests/experiment/test_gbdt.py b/tests/experiment/test_gbdt.py
index 448ab5d..10a47aa 100644
--- a/tests/experiment/test_gbdt.py
+++ b/tests/experiment/test_gbdt.py
@@ -376,19 +376,3 @@ def test_experiment_sample_submission_multiclass():
         log_loss_default = log_loss(y_test, np.full((len(y_test), 5), 0.2), labels=[0, 1, 2, 3, 4])
         assert log_loss_trianed < log_loss_default
 
-
-def test_find_best_parameter():
-    params = {
-        'objective': 'binary',
-        'metrics': 'auc',
-        'n_estimators': 1000
-    }
-    X, y = make_classification_df(2048, class_sep=0.7)
-    X_train, X_test, y_train, y_test = train_test_split(X, y)
-
-    best_params = find_best_lgbm_parameter(params, X_train, y_train, cv=5)
-
-    result_base = experiment_gbdt(params, X_train, y_train, eval_func=roc_auc_score)
-    result_opt = experiment_gbdt(best_params, X_train, y_train)
-
-    assert result_opt.metrics[-1] >= result_base.metrics[-1]