Hi..
Typically the transformer will keep the voltage output steady despite the voltage input... So I think you'll be OK...
Here's an option (
http://tinyurl.com/dfn5mr) that I bought so that I could just use the same USB chargers that I have already.. There's also some nice retractable ones out there that you probably can get in Japan, do a search for "Power and Data Cable for PSP"...
Hope this helps..