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