Earlier quoted context omitted.
You can't start a side business on a regular work visa. You know YC's question "How have you ever hacked a non-technical system?" This is my answer to it. You can start a side business on a regular work visa if the character of your business matches your status of residence and you don't intensify the business to the point where it is undeniable that it is a business as opposed to employment, e.g. by hiring full-time…
wait, as a non-citizen of Japan you are allowed to sponsor yourself for a work visa? That sounds, uhh, strange.
I don't know how much difficult it is start your own business in Japan, for this Patrick is clearly the most knowledgeable person here, but once it's done, you just need to give yourself a salary to be able to renew your visa.