Until recently, Japan had been unique in having no specialized legal provision for mentally ill criminal offenders. After longterm debate, fresh legislation was passed at last in 2003 to deal with the offenders who committed a felony such as homicide, arson, robbery, rape, indecent assault and injury under insane conditions by which they were judged to be criminally irresponsible or to have diminished responsibility. This new law came into effect in mid 2005 as the Act for the Medical Treatment and Supervision of Insane Persons who Caused Serious Harm (often abbreviated as ‘Medical Treatment and Supervision Act’), which is to be reformed according to necessity in 2010. Until 1st of November 2008, this law had already been administered to 1,264 cases. Considering that this act has not yet been widely known abroad and thus it tends to evade criticism from international forensic psychiatric points of view, it appears timely to survey this act and related mental health care systems in Japan by which mentally ill patients with criminal conduct are treated. In this article, we review the present status of how to manage mentally ill offenders and violent psychiatric patients within the framework of Japanese law system, focusing on clinical and practical problems experienced by general psychiatrists engaged in treatment of these patients. Our hope is that this review presents a clue to gather international peer critical opinions leading to reconsideration and revision of the present systems, resulting in contribution to better treatment of all patients with psychiatric disorders.