diff --git a/optuna_dashboard/_custom_plot_data.py b/optuna_dashboard/_custom_plot_data.py
index a88fc0a4..a4dfa4af 100644
--- a/optuna_dashboard/_custom_plot_data.py
+++ b/optuna_dashboard/_custom_plot_data.py
@@ -53,7 +53,7 @@ def save_plotly_graph_object(
     Returns:
         The graph object ID.
     """
-    if graph_object_id is not None and not is_valid_html_name(graph_object_id):
+    if graph_object_id is not None and not is_valid_graph_object_id(graph_object_id):
         raise ValueError("graph_object_id must be a valid HTML id attribute value.")
 
     storage = study._storage
@@ -119,18 +119,16 @@ def concat_plot_data(plot_data_attrs: dict[str, str], key_prefix: str) -> str:
     return "".join(plot_data_attrs[f"{key_prefix}{i}"] for i in range(len(plot_data_attrs)))
 
 
-def is_valid_html_name(graph_object_id: str) -> bool:
+def is_valid_graph_object_id(graph_object_id: str) -> bool:
     if len(graph_object_id) == 0:
         return False
 
-    # Must begin with a letter [A-Za-z]
-    if not ("a" <= graph_object_id[0] <= "z" or "A" <= graph_object_id[0] <= "Z"):
-        return False
-
-    # Can only contain letters [A-Za-z], numbers [0-9], hyphens ("-"), underscores ("_"), colons, and periods.
+    # Can only contain letters [A-Za-z], numbers [0-9], hyphens ("-"), underscores ("_"),
+    # colons, and periods.
     if not all(
         "a" <= c <= "z" or "A" <= c <= "Z" or "0" <= c <= "9" or c in ("-", "_", ":", ".")
         for c in graph_object_id[1:]
     ):
         return False
+    # Unlike HTML id attribute, graph object id can begin with a letter [A-Za-z]
     return True
diff --git a/python_tests/test_custom_plot_data.py b/python_tests/test_custom_plot_data.py
index f73f7097..3dcfc856 100644
--- a/python_tests/test_custom_plot_data.py
+++ b/python_tests/test_custom_plot_data.py
@@ -65,22 +65,22 @@ def test_update_plotly_graph_object() -> None:
 @pytest.mark.parametrize(
     "name",
     [
+        "0",
         "a",
         "a1-:_.",
     ],
 )
-def test_is_valid_html_name(name):
-    assert custom_plot_data.is_valid_html_name(name)
+def test_is_valid_graph_object_id(name: str) -> None:
+    assert custom_plot_data.is_valid_graph_object_id(name)
 
 
 @pytest.mark.parametrize(
     "name",
     [
-        "0",
         "a,",
         "a b",
         "aあいうえお",
     ],
 )
-def test_is_invalid_html_name(name):
-    assert not custom_plot_data.is_valid_html_name(name)
+def test_is_invalid_graph_object_id(name: str) -> None:
+    assert not custom_plot_data.is_valid_graph_object_id(name)