As a recent college graduate and a current English teacher in Japan, I can affirm that you need a bachelor’s degree and a passport. If you are recruited from outside of Japan, the company who hires you will likely help you with your visa. However, as is the case for most things, meeting the minimum qualifications (i.e. having a degree) does in no way guarantee you a position at your desired place of employment. Teaching experience, even volunteer work, is a plus. People skills and a firm grasp of the English language are implicit in the job description.

If you’re serious about teaching English in Japan, this website helped me a lot. With increasing need for English in the workplace, and heightened requirements for English in schools (students will start in Elementary, and the average high school student will now spend more time in an English classroom than in Math or Japanese), I can say with confidence that the job you want will still be around in four or five years.