Depending on what you get up to in Japan, you may lie awake at night pondering important questions like "Who can arrest me, and why?"
Perhaps not; maybe you don't run a Bitcoin exchange or have Oxycontin delivered by international post. Still, you might want to know anyway, just in case.
The answer to "Who can arrest me?" is, actually, "anyone." Japan's Code of Criminal Procedure (CCP) allows for what in some countries are called "citizen's arrests" of offenders for crimes in progress. You don't even have to be a citizen.