If you've ever wondered whether you can keep your visa if you quit or lose your job, the answer isn't necessarily black and white. I've heard some people say that the visa is yours to keep, but that isn't always true, although it can be for some people.
Read the answer, plus an answer on if there's a maximum hour work week for those with Instructor visas, here:
How would changing jobs affect my visa? | Japan Times (Jul. 31, 2012)