Japan uses 100V at 50/60Hz as its standard household electrical supply. If you're moving appliances between Japan and North America (120V/60Hz), both voltage and frequency are worth checking before you plug anything in.
Japan runs a split grid: 50/60Hz depending on region — this isn't a typo. The best-known example is Japan itself, where the Tokyo area runs 50Hz and the Osaka area runs 60Hz, a legacy of which country's equipment its power company bought from in the 1890s (see the full history). If you're bringing equipment into Japan, confirm the frequency for the specific region you'll be in before assuming either standard — a device that's fine in one part of the country may not be in another.
Since Japan runs at 100V, a US-spec 120V appliance would need its voltage stepped up, and a local appliance from Japan would need its voltage stepped down to run on North American 120V power.
A plug adapter only changes the physical shape of the plug — it doesn't change voltage or frequency at all. If the appliance isn't rated for Japan's 100V/50/60Hz supply, a plug adapter alone can damage it.
No — check the rating label on the back or bottom of each appliance. Many modern electronics (phone chargers, laptops) are dual-voltage and frequency-tolerant. Motor-driven and timing-based appliances are the ones that need real conversion.