Guidelines for treating depression often recommend continuing antidepressants at least for 6 months after remission. Whether this recommendation is implemented in daily practices represents a serious concern. We aimed to examine adequacy of continuation and maintenance treatment in Japan. A naturalistic prospective follow-up study with mood disorders was undertaken in 23 psychiatric departments from all over Japan. A total of 95 patients diagnosed with major depression were followed up every month until treatment termination and every 6 months thereafter. In this study, the cohort received 45.1 (SD = 64.7) mg of imipramine or equivalent per day during continuation phase, and about 74% were prescribed inadequate doses, i.e. less than 75 mg/day. At maintenance phase immediately before relapse, average dosage was 42.0 (SD = 74.7) mg/day and 83% were prescribed inadequate doses. There is gross under-treatment of depression during continuation and maintenance phases in Japan.