Compatibility summary
Your Japan plug may fit in Marshall Islands outlets, as both use at least one common plug type.
Plug comparison
Japan plug types: A B
Marshall Islands plug types: A B
Both countries share at least one plug type (A B and A B), so some plugs may work without an adapter. Check your appliance.
Voltage comparison
Japan: 100V, 50Hz
Marshall Islands: 120V, 60Hz
Voltage difference is within a range many devices tolerate; check your appliance label.