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