just spoke with a friend of mine that came back from japan after a short study abroad as well as a co-op with a japanese company. she told me that basically if you head to japan, you've got 90 days to wander about, any longer and if you wish to have a job you're going to have to have a work visa. just like in the us, some company there is going to have to sponsor you in order for you to obtain this. also, it's much more likely that a company would sponsor you before you left for japan (as opposed to seeking sponsorship after you got there). if you don't have a work visa then you probably shouldn't expect to be in/around tokyo or other larger cities and working, more likely a small village doing some oddjobs where the folks don't care that you don't have a visa.

just thought this would be a useful fyi to you