diff --git a/optuna_dashboard/_custom_plot_data.py b/optuna_dashboard/_custom_plot_data.py
index d669d4ad..a88fc0a4 100644
--- a/optuna_dashboard/_custom_plot_data.py
+++ b/optuna_dashboard/_custom_plot_data.py
@@ -48,10 +48,14 @@ def save_plotly_graph_object(
             The plotly's graph object to save.
         graph_object_id:
             Unique identifier of the graph object. If specified, the graph object is overwritten.
+            This must be a valid HTML id attribute value.
 
     Returns:
         The graph object ID.
     """
+    if graph_object_id is not None and not is_valid_html_name(graph_object_id):
+        raise ValueError("graph_object_id must be a valid HTML id attribute value.")
+
     storage = study._storage
     study_id = study._study_id
 
@@ -113,3 +117,20 @@ def split_plot_data(plot_data_str: str, key_prefix: str) -> dict[str, str]:
 
 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:
+    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.
+    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
+    return True
diff --git a/python_tests/test_custom_plot_data.py b/python_tests/test_custom_plot_data.py
index 4d01af96..f73f7097 100644
--- a/python_tests/test_custom_plot_data.py
+++ b/python_tests/test_custom_plot_data.py
@@ -3,6 +3,7 @@ from __future__ import annotations
 import optuna
 from optuna_dashboard import _custom_plot_data as custom_plot_data
 from optuna_dashboard import save_plotly_graph_object
+import pytest
 
 
 def get_dummy_study() -> optuna.Study:
@@ -59,3 +60,27 @@ def test_update_plotly_graph_object() -> None:
     plot_data_dict = custom_plot_data.get_plotly_graph_objects(study_system_attrs)
     assert len(plot_data_dict) == 1
     assert plot_data_dict[graph_object_id] == plot_data.to_json()
+
+
+@pytest.mark.parametrize(
+    "name",
+    [
+        "a",
+        "a1-:_.",
+    ],
+)
+def test_is_valid_html_name(name):
+    assert custom_plot_data.is_valid_html_name(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)