If an individual has strong ties to Japan and a valid visa status, he/she may be allowed to remain in Japan after receiving a suspended sentence, if a Japanese national acts as his/her sponsor with the Immigration authorities. This typically occurs in cases where the prisoner has a Japanese spouse. Those without strong ties to Japan and whose visa status has expired would be deported after their conviction.
Similarly, an individual convicted and sentenced to actual imprisonment would have the same options after release from prison. Those with strong ties and a willing sponsor in Japan could be permitted to remain while others would be deported after their release from prison. Deportation in Japan is either at the deportee's own expense, or, after a wait of perhaps an additional six to eight weeks in custody after the conclusion of a prison sentence, at the expense of the Government of Japan.
Under some circumstances, Americans may soon be able to serve all or part of their sentence in the U.S. instead of in a Japanese prison. Follow this link to learn more about Prisoner Transfer.