Are you a foreigner living in Japan or are you Japanese? There's a BIG difference in marketability. Some American pro's come here to do endorsments and guest posing. I advise you buy a Japanese Bodybuilding magazine and take a look.
The IFBB (International Federation of Bodybuilders) has rules set up accordingly. They've got a big pretty website you can browse. However, to make a career as a bodybuilder isn't an easy thing, no matter where you are. Contest prize money isn't much, most money in the industry is made in supplement endorsments. And, most of that money is in the states. You might not believe this, but even someone like good 'ol Bob Sappu would have a hard time as a competitive bodybuilder in the states.
I had a Japanese friend who was into body building, but the best job he could get was working in that famous gym in Tokyo, as a trainer for the gym's customers. Does anybody know the name of that gym in Tokyo? It had a gaijin's name, if I remember.