Compatibility summary
You will need a travel adapter in Niger; 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
Niger plug types: C E
There is no common plug type between Japan (A B) and Niger (C E). You need a travel adapter.
Voltage comparison
Japan: 100V, 50Hz
Niger: 230V, 50Hz
Voltage converter recommended (Japan 100V vs Niger 230V).