OBJECTIVES: Depression is one of the most common mental disorders. However, depressed patients are underdiagnosed and undertreated in the general population. We aimed to evaluate the prevalence of undiagnosed depression in the general Japanese population, healthcare utilization and somatic symptoms among undiagnosed depressed patients. METHODS: In a cross-sectional study, a population-weighted random sample from a nationally representative panel of adults was used to estimate health care utilization based on a prospective health diary recorded over one month. A baseline, pre-diary questionnaire was used to capture demographic data and symptoms. Depression was defined as a score of >9 on the Patient Health Questionnaire (PHQ-9) and undiagnosed depressive patients were defined as those with a score of >9 on the PHQ-9 who did not report the treatment of depression in hospital. RESULTS: A total of 2,113 people were included in this study (mean age, 52 years (SD, 18); 1,236 (58.5%) women.) The prevalence of depression was 16.2% (n=343), among whom 264 (12.5%) were undiagnosed. Incidence rate of any symptom over a one month period among non-depressive, undiagnosed depressive, and depressive patients was 3.29, 4.40, and 4.92, respectively (p=0.1). The most common symptoms were headache (37.2%), followed by back pain (35.5%) and diarrhea (20.0%). Comparing the non-depressive, undiagnosed depressive, and depressive patient groups, the symptoms showing the largest difference in prevalence were nausea (3.7%, 6.8%, and 15.2%, respectively p<0.01) and gastric pain (6.0%, 10.6%, and 12.7%, respectively p<0.01). Healthcare utilization rates per month were 31.7%, 38.6%, and 31.6% (p=0.08) for non-depressive, undiagnosed depressive, and depressive patients, respectively. CONCLUSIONS: Depressive disorders are prevalent in Japan and about 80% was undiagnosed. Screening for depression might be useful for those presenting with many symptoms including those of the gastrointestinal system.