“The people are so law-abiding” is a common Western trope about Japan. It is mostly true, of course, but it probably also helps that information about who has broken the law is kept tightly under wraps.
The front end of the nation’s criminal justice system gets a lot of bad press due to a penchant for prolonged pre-charge detention periods with limited access to counsel that help generate conviction rates well above 90%. However, once you submit to authority and demonstrate suitable contrition things improve greatly. Suspended sentences are common and even if you are sent to prison, the system generally tries to get you out as soon as possible.
After fines, the most common punishment meted out is imprisonment with labor. This reflects the rehabilitative focus of Japanese prisons. The idea is that working in prison factories gives convicts structure and skills to make them more employable after release.