I can only speak for German citizens. And they can go to Japan for 90 days without visa, they can extend the validity up to 180 days. But if you are more than 180 days in Japan you will need a visa.
There are infos about the visa and other documents:
Immigration to Japan
And if you want to open your own store or restaurant, I can imagine it's very difficult to get the necessary visa.
I know the situation from Iceland. And there you need a residence (contract), medical insurance and the contract with your employer (so it's not too easy to be an alien self-employed) and some documents more.
And in Japan the situation is not easier. Necessary is a university degree or several years of working experience.