Background: Despite numerous changes in the mental health care system in Japan in 2000's, little is known about changes in the prevalence or treatment rates of mental disorders. Methods: The World Mental Health Japan (WMHJ) 2nd Survey was a nationally representative face-to-face household survey of residents aged 20–75 years old conducted between 2013 and 2015. We compared the findings with those of an earlier study, the first WMHJ (WMHJ1) survey, conducted in 2002–2006. Results: Overall, 2450 residents completed the interview. Lifetime prevalence of any common mental disorder was 22%, with high prevalence of alcohol abuse (15.1%). Twelve-month prevalence rates of any common mental disorder and major depressive disorder were 5.2% and 2.7%, respectively. Severe cases comprised 24% of 12-month disorders. Lifetime prevalence of any common mental disorder was greater for males. The persistence of any common mental disorder was greater for females. Proportion of those with 12-month disorders who sought treatment was 34%. Mental health care use was the most prevalent among 12-month cases. Twelve-month prevalence of any common mental disorder was similar to that of the WMHJ1 while the proportion of seeking treatment increased for disorders with moderate severity. Limitations: Institutionalized people were not included. The lower response rate might also limit the interpretation of the findings. Conclusions: The study found lower prevalence of mental disorders in Japan than in Western countries in the mid 2010's, suggesting that the prevalence of mental disorders remained stable in the last decade in Japan. Treatment rate increased for those with moderate disorders, which might reflect the government's attempt to raise people's awareness of mental health.