Your 360 is a US 360? If so you should have no problems regarding how much power is supplied. Japan uses 100v outlets and the USA uses 110v, so they are pretty much interchangeable. Though as somebody else mentioned the Japanese outlets do not have the 3rd pin (the ground plug), so you may need an adapter for it (3 pin -> 2 pin adapter). Or you could buy a Japanese 360 cord ;-p
I've been using my US PS2 in Japan for the past 2 years and I've had no problems with it. My 360 is a Japanese 360. :)
Just curious, but where will you be living in Japan?
starcraft: "I and every PS3 fanboy alive are waiting for Versus more than FFXIII.
Me since the games were revealed, the fanboys since E3." 
Skeeuk: "playstation 3 is the ultimate in gaming acceleration"
