This study was designed to reveal the current status of the rehabilitation support system for patients with Illegal Substance Use Disorder (ISUD). From among 465 patients who had been admitted to a psychiatric hospital in Tokyo within the past 10 years by order of the prefectural governor, 65 patients with ISUD were selected for inclusion in this study. Based on whether or not the person was arrested at the time of discharge, whether or not urine drug monitoring was ordered, and the results of the monitoring, each subject was classified into one of the following four types: 1) Arrested; 2) Not arrested and no urine drug monitoring; 3) Not arrested and positive urine drug monitoring results; and 4) Not arrested and negative urine drug monitoring results. In Group 1, every subject underwent urine drug monitoring prior to an involuntary examination; however, even though 10 percent of the subjects in this study were found to have positive results on urine drug monitoring, none of them were arrested. Moreover, 40 percent of the study subjects were not subjected to urine drug monitoring, and about 30 percent of non-arrested subjects were shown not to have used any illegal substances. Based on these results, it appears to be ideal for patients in Group 1 to apply to a diversion program, followed by medical treatment for addiction. To avoid the elimination of patients from medical services due to the vagueness of the classifications, whether or not judicial administration is required at the time of police intervention should be clearly and appropriately clarified for patients in Groups 2 and 3. Patients in Group 4 may experience a relapse of psychiatric symptoms, even if they do not use illegal substances; therefore, it is necessary for designated hospitals to perform medical treatment interventions responsibly for both endogenous psychosis and substance abuse, and to collaborate with appropriate social support facilities within the community regarding the medical discharge of such patients.