Japan → United States
Japan plugs
Type A
Type BUnited States plugs
Type AType BShared plug types: A, B
Compatibility: Compatible
Voltage — Japan 100V — United States 120V
Voltage difference is small. Most electronics will work safely.
Device compatibility (phones)
Phones almost always support 100–240V, so yours should work safely. Check your charger label to confirm.
You likely only need a plug adapter (if socket shapes differ). Voltage is close between these countries.