diff --git a/t4_devkit/tier4.py b/t4_devkit/tier4.py
index a4787b0..7b5a93c 100644
--- a/t4_devkit/tier4.py
+++ b/t4_devkit/tier4.py
@@ -41,7 +41,7 @@
         Visibility,
     )
 
-__all__ = ["DBMetadata", "load_metadata", "Tier4"]
+__all__ = ["DBMetadata", "load_metadata", "load_table", "Tier4"]
 
 
 @define
@@ -92,6 +92,29 @@ def load_metadata(db_root: str, revision: str | None = None) -> DBMetadata:
     return DBMetadata(data_root=data_root, dataset_id=dataset_id, version=version)
 
 
+def load_table(annotation_dir: str, schema: SchemaName) -> list[SchemaTable]:
+    """Load schema table from a JSON file.
+
+    If the schema is optional and there is no corresponding JSON file in dataset,
+    returns empty list.
+
+    Args:
+        annotation_dir (str): Path to the directory of JSON annotation schema files.
+        schema (SchemaName): An enum member of `SchemaName`.
+
+    Returns:
+        Loaded table data saved in `.json`.
+    """
+    filepath = osp.join(annotation_dir, schema.filename)
+    if not osp.exists(filepath) and schema.is_optional():
+        return []
+
+    if not osp.exists(filepath):
+        raise FileNotFoundError(f"{schema.value} is mandatory.")
+
+    return build_schema(schema, filepath)
+
+
 class Tier4:
     """Database class for T4 dataset to help query and retrieve information from the database."""
 
@@ -152,27 +175,29 @@ def __init__(
             print("======\nLoading T4 tables...")
 
         # assign tables explicitly
-        self.attribute: list[Attribute] = self.__load_table__(SchemaName.ATTRIBUTE)
-        self.calibrated_sensor: list[CalibratedSensor] = self.__load_table__(
-            SchemaName.CALIBRATED_SENSOR
+        self.attribute: list[Attribute] = load_table(self.annotation_dir, SchemaName.ATTRIBUTE)
+        self.calibrated_sensor: list[CalibratedSensor] = load_table(
+            self.annotation_dir, SchemaName.CALIBRATED_SENSOR
         )
-        self.category: list[Category] = self.__load_table__(SchemaName.CATEGORY)
-        self.ego_pose: list[EgoPose] = self.__load_table__(SchemaName.EGO_POSE)
-        self.instance: list[Instance] = self.__load_table__(SchemaName.INSTANCE)
-        self.keypoint: list[Keypoint] = self.__load_table__(SchemaName.KEYPOINT)
-        self.log: list[Log] = self.__load_table__(SchemaName.LOG)
-        self.map: list[Map] = self.__load_table__(SchemaName.MAP)
-        self.object_ann: list[ObjectAnn] = self.__load_table__(SchemaName.OBJECT_ANN)
-        self.sample_annotation: list[SampleAnnotation] = self.__load_table__(
-            SchemaName.SAMPLE_ANNOTATION
+        self.category: list[Category] = load_table(self.annotation_dir, SchemaName.CATEGORY)
+        self.ego_pose: list[EgoPose] = load_table(self.annotation_dir, SchemaName.EGO_POSE)
+        self.instance: list[Instance] = load_table(self.annotation_dir, SchemaName.INSTANCE)
+        self.keypoint: list[Keypoint] = load_table(self.annotation_dir, SchemaName.KEYPOINT)
+        self.log: list[Log] = load_table(self.annotation_dir, SchemaName.LOG)
+        self.map: list[Map] = load_table(self.annotation_dir, SchemaName.MAP)
+        self.object_ann: list[ObjectAnn] = load_table(self.annotation_dir, SchemaName.OBJECT_ANN)
+        self.sample_annotation: list[SampleAnnotation] = load_table(
+            self.annotation_dir, SchemaName.SAMPLE_ANNOTATION
         )
-        self.sample_data: list[SampleData] = self.__load_table__(SchemaName.SAMPLE_DATA)
-        self.sample: list[Sample] = self.__load_table__(SchemaName.SAMPLE)
-        self.scene: list[Scene] = self.__load_table__(SchemaName.SCENE)
-        self.sensor: list[Sensor] = self.__load_table__(SchemaName.SENSOR)
-        self.surface_ann: list[SurfaceAnn] = self.__load_table__(SchemaName.SURFACE_ANN)
-        self.vehicle_state: list[VehicleState] = self.__load_table__(SchemaName.VEHICLE_STATE)
-        self.visibility: list[Visibility] = self.__load_table__(SchemaName.VISIBILITY)
+        self.sample_data: list[SampleData] = load_table(self.annotation_dir, SchemaName.SAMPLE_DATA)
+        self.sample: list[Sample] = load_table(self.annotation_dir, SchemaName.SAMPLE)
+        self.scene: list[Scene] = load_table(self.annotation_dir, SchemaName.SCENE)
+        self.sensor: list[Sensor] = load_table(self.annotation_dir, SchemaName.SENSOR)
+        self.surface_ann: list[SurfaceAnn] = load_table(self.annotation_dir, SchemaName.SURFACE_ANN)
+        self.vehicle_state: list[VehicleState] = load_table(
+            self.annotation_dir, SchemaName.VEHICLE_STATE
+        )
+        self.visibility: list[Visibility] = load_table(self.annotation_dir, SchemaName.VISIBILITY)
 
         # make reverse indexes for common lookups
         self.__make_reverse_index__(verbose)
@@ -217,27 +242,6 @@ def bag_dir(self) -> str:
         """Return the path to ROS bag directory."""
         return osp.join(self.data_root, "input_bag")
 
-    def __load_table__(self, schema: SchemaName) -> list[SchemaTable]:
-        """Load schema table from a json file.
-
-        If the schema is optional and there is no corresponding json file in dataset,
-        returns empty list.
-
-        Args:
-            schema (SchemaName): An enum member of `SchemaName`.
-
-        Returns:
-            Loaded table data saved in `.json`.
-        """
-        filepath = osp.join(self.annotation_dir, schema.filename)
-        if not osp.exists(filepath) and schema.is_optional():
-            return []
-
-        if not osp.exists(filepath):
-            raise FileNotFoundError(f"{schema.value} is mandatory.")
-
-        return build_schema(schema, filepath)
-
     def __make_reverse_index__(self, verbose: bool) -> None:
         """De-normalize database to create reverse indices for common cases.