And even if you are married to a Japanese, you'll need to have lived in Japan for a number of years before applying. And, after all that, not everyone is approved.

There is another option. You can get what is called a shokutaku visa (it's a subset of the bunka koryu visa, which is what you get as an English teacher).

What you get is signed statements from a number of companies promising to use your services up to a certain monthly (or yearly) minimum amount. When I did this the total was ¥200,000, it may be more now. In practice nobody checks up on the exact numbers as long as you do indeed earn this amount or more in total.

If you can get such promissory statements you can legally work in Japan without a sponsor. Just drop in at the immigration office and ask, they're quite helpful. It's a lot easier (and cheaper!) than getting an investor/manager visa.