Advances in pharmacological and psychological treatment in today’s psychiatry have enabled many people with mental disorders to maintain an acceptable quality of life, while continuing to participate in society in their communities. However, the mental health service system for severe mental illness in Japan has yet to completely shift away from a hospitalization-centered care model. At the same time, expectations for psychiatric care have become broaden due to recent social recognition of the societal burden of common mental disorders such as depression and anxiety. However, it is clear that psychiatric services alone cannot meet these growing demands. To address these challenges, health services research (HSR) is essential. HSR is an academic discipline that scientifically evaluates whether people with health needs can access effective care, whether that care is appropriately implemented, and whether it yields meaningful benefits. Based on these assessments, it proposes strategies for improving service systems.