Compatibility summary
You will need a travel adapter in Hong Kong; plug types used there differ from Japan. Voltage differs by more than 20% — a voltage converter may be needed for some appliances.
Plug comparison
Japan plug types: A B
Hong Kong plug types: G
There is no common plug type between Japan (A B) and Hong Kong (G). You need a travel adapter.
Voltage comparison
Japan: 100V, 50Hz
Hong Kong: 220V, 50Hz
Voltage converter recommended (Japan 100V vs Hong Kong 220V).