Not overly concerned... yet.

[ramchop]

Just what do they do to gaijin who don't pay taxes?

[Taro Toporific]

Just what do they do to gaijin who don't pay taxes?

Not fvrking much. The J-tax guys' main threat with Japanese is not having a "family register" registered. To a FG, they try to nail ya when you renew your visa or buy property. I went 6 years without paying, and then renewed my visa through a 3rd country embassy.

Hint: FG are invisble to the Japanese feudal "family register" system---enjoy it.

[Taro Toporific]

Just what do they do to gaijin who don't pay taxes?

DANG! The devil is in the details...

Assumption: You are 1) single and 2) self-employed.
Tax non-payment is minor concern since the tax people don't know or care about alien beings.
(If you are married, you have a "Pet Licence" version of a Japanese family register and this reduces your tax "flexiblity.")

Assumption: You are willing to "play the game."

To avoid Ward tax, you need to move your ward every 3 years to cover your trail (and allow getting a Visa without making much of any ward tax payment).

To avoid national tax, you need 1) to not report income and 2) to have a clean paper trail (no obvious bank accounts in your real name). If you're nailed with national tax for some rare reason, you can leave and come back as a "clean person." Ideally, you should be paid income in a tax shelter country (most TIME magazine's FG employees in Tokyo are paid via Hong Kong's flat-tax for example).

REMEMBER: Paying some trivial taxes is often a good idea in terms of visa getting, Japanese credit cards, health insurance, etc. "Low profile" is often safer than "no profile".