If you are returning from an extended stay in Japan, you don't have to leave your appliances behind. However, you must buy the proper adaptor for American current (often called a "step-down" adaptor). Although Japanese plugs are two-pronged like American plugs, the current and the frequency at which electricity runs (known as "Hertz") differ in America, and must be accommodated for both your safety and the longevity of your Japanese-made machines.
Read the label for your Japanese machine. It more than likely is set to use 100 volts and 50 Hz. This means that if you do not buy an adaptor to adjust the voltage, your machine will get too much juice from American currents, which run at 120 volts and 60 Hz.
Search for an adaptor if you are still in Japan. It is easier--by far--to buy the proper adaptor from a travel store or resource in Japan, than to try and buy the adaptor in America. If you haven't left Japan, try your local stores. If you are in the military, they should be available at your base's local PX.
Find an adaptor if you are already in America. The quickest and easiest way to find one is to shop online (see Resources for a link to an example).
Plug the adaptor into the outlet and then plug in your machine.
Be specific with the supplier of the converter. The adaptor must be able to handle both the voltage and the Hertz conversion to work properly. Measure the cost of bringing an appliance from Japan. With the cost of purchasing an adaptor, it is generally much cheaper to just buy a replacement appliance in the United States.
Using a machine that is set for a different voltage than you have it plugged into is a fire hazard.