The number of patients with mental disorders is increasing all over the world and they have a high prevalence of physical complications. To analyze and compare the cost of mental disorders with the total medical costs such as mental disorders and physical disorders, we analyzed the cost that patients with mental disorders incurred. We investigated the distribution of medical costs and the characteristics of diseases by using the health insurance claims of people in City A, Japan from March 2013 to February 2014. The subjects had one or more of 4 mental disorders: schizophrenia, alcohol related disorders, bipolar disorder, and depressive episode. As a result, the total number of patients who met inclusion criteria per year was 7403 (6522 outpatients and 881 inpatients). It was revealed that the hospitalization rate of patients with mental disorders increased with age, and many inpatients stayed in hospital for a long time. Also, it was revealed that many patients with mental disorders were in complicated condition with more than one mental or physical disorders and incurred medical costs for these comorbidities. In conclusion, this analysis indicated that many patients with mental disorders switch from attending the outpatient department to hospitalization as they become older. Further, they incurred a lot of medical expenses for complication management. To improve their QOL, comprehensive assessment of their mental/ physical health, self-management education, coordination of services, and support for decision making regarding treatment are necessary.