Aim: Major depression is expected to become the leading contributor to disease burden worldwide by 2020. Previous studies have shown that the societal cost of depression is not less than that of other major illnesses, such as cardiovascular diseases or AIDS. Nevertheless, the cost of depression in Japan has never been examined. The goal of the present study was to estimate the total cost of depression in Japan and to clarify the characteristics of this burden. Methods: A prevalence-based approach was adopted to measure the total cost of depression. The total cost of depression was regarded as being comprised of the direct cost, morbidity cost and mortality cost. Diagnoses included in this study were depressive episodes and recurrent depressive disorder according to the ICD-10 or major depressive disorder according to the DSM-IV. Data were collected from publicly available statistics and the World Mental Health Japan Survey database. Results: The total cost of depression among adults in Japan in 2005 was estimated to be ¥2.0 trillion. The direct cost was ¥0.18 trillion. The morbidity cost was ¥0.92 trillion, while the mortality cost was ¥0.88 trillion. Conclusion: The societal costs caused by depression in Japan are enormous, as in other developed countries. Low morbidity costs and extremely high mortality costs are characteristic in Japan. Effective interventions for preventing suicide could reduce the societal costs of depression. © 2011 Japanese Society of Psychiatry and Neurology.