You cannot work legally in Japan without obtaining a work visa, especially if you are teaching. If you work in any field without one, you will be working illegally and subject to prosecution and deportment. Work visas in almost any field require having a degree in that field or in an obviously related field and being sponsored by a company which will guarantee your income.

On the Other: Work Experience and Marriage Count, Too

If you are the spouse or the child of a Japanese national or a permanent resident of Japan, you can obtain work visas without sponsorship. Companies may still ask for degrees, however. Self-sponsorship is possible, but requires you already living and supporting yourself in Japan for some time. It is also possible to get a work visa without a college degree if you have many years of experience in the field.

Bottom Line

You will need some sort of work visa to get a job in Japan. The easiest way to get one is to have a degree and experience in your field. However, there are provisions for spouses or children of Japanese nationals to obtain work visas without degrees. You may also be very fortunate and find a company willing to sponsor your visa without a degree if you have many years of experience.

Source:

Transitions Abroad: Teaching English in Japan

Visa to Japan: Visa Status

Gaijin Pot: Visas