You need a visa for Sierra Leone if you have a Japanese passport
Visa options for Sierra Leone
Electronic visitor visa
Travellers need a visa to visit Sierra Leone for Business or Tourism. A Sierra Leone eVisa is the fastest and easiest way of obtaining a visa for Sierra Leone because travellers can submit their applications and supporting documents online. Once the application is approved, travellers receive their eVisa confirmation electronically. A valid visa is required to board the flight. If the visa is expired or doesn’t match the traveller’s passport information, travellers will not be able to board their flight.