<p>Background: Mental disorders sharply increase in adolescence. Since most of adolescents spend a major portion of their time at school, teachers are in a good position to give them key supports for their mental health difficulties. For this, mental health literacy (MHL) needs to be improved in teachers. We have developed a MHL training program for teachers, with an original 50-min video (DVD)</p><p>Objectives: To examine whether the program which we have developed improve teachers' MHL.</p><p>Methods: Study design was case-series. Forty-four teachers participated in the study and watched the DVD. Teachers' MHL was evaluated using a self-report questionnaire before and after the program. The questionnaire quantitatively measured main components of MHL, including (1) knowledge about mental disorders and their treatments, (2) ability to recognize specific mental disorders, (3) attitudes towards students with depressive symptoms, and (4) intention to help students with the symptoms.</p><p>Results: All main components of MHL were significantly improved after the intervention. Large effect was observed on teachers' knowledge about mental disorders and treatments (d=0.85). Medium effect was observed on attitudes toward students with depressive symptoms and intention to help students with depressive symptoms (d=0.55 and d=0.67, respectively).</p><p>Conclusion: Teachers' MHL can be improved by the short-video MHL program which is conducted without recruiting human resources from outside the school.</p>