Travelers’ Essentials   Plugs and Adapters in Japan and S. Korea Japan The voltage in Japan is 100V. US devices come with 2 prong plugs should work without an adapter or a power converter in Japan, but If you use a three plong cabel, you will need a 3-pin plug to a 2-pin. Typical 2-pin Japanese Outlet 3-pin plug to a 2-pin Korea The voltage in Korea is 220V. US devices come with 2 prong plugs cannot be used without an adapter in Korea. You will need adaptors with or without USB ports. Typical Power sockets in South Korea