Welcome to TCC.
I use a Garmin eTrex Legend, with Garmin MapSource software for managing the maps. There is an English map available of Japan from www.uud.info which is a detailed as you need; cost is 8400 Yen.
You need to be a bit careful buying a GPS here, as the Garmin ones appear to have some kind of region lock on them. When I bought the Mapsource software I got the official Garmin Japan Topographic map with it. But, the maps have to be 'unlocked' through Garmin's website, and the official Japan map won't unlock for my eTrex, which I bought in USA. There may also be issues with display of Japanese characters on non-Japanese units.
GPS units here also seem to be at least double the price of the same unit in UK/US.
Unless you can read Kanji, my recommendation is to get a GPS sent from USA/Europe and load the www.uud.info map onto it.
this thread is just what I'm looking for. I've one question, which maybe Alan can answer. On the Garmin site, there's two new ones called the Edge 605 and Edge 705 which look very swish. Do these look like good buys (assuming they work in Japan)?
Edit: Just noticed the price difference between these and Alan's etrex legend...