The electrical current in Japan is 100 volts, 50 cycles alternating current (AC) in eastern Japan, and 100 volts, 60 cycles in western Japan. The United States runs on 110-volt, 60-cycle AC current. Wall outlets in Japan accept plugs with two flat prongs, as in the United States, but do not accept U.S. three-prong plugs.
Consider making a small investment in a universal adapter, which has several types of plugs in one lightweight, compact unit. Most laptops and mobile phone chargers are dual voltage (i.e., they operate equally well on 110 and 220 volts), so require only an adapter. These days the same is true of small appliances such as hair dryers. Always check labels and manufacturers' instructions to be sure. Don't use 110-volt outlets marked for shavers only for high-wattage appliances such as hair dryers.