Objectives: Long working hours have the possibility to influence human health. In Japan, it is well known that teachers have long working hours, and the number of leaves of absence due to mental disorders among public school teachers increased from 2,687 in 2002 to 4,960 in 2012. The aim of this study was to investigate the association between long working hours and psychological distress among school teachers. Methods: This cross-sectional study was conducted from mid-July to September in 2013 in Hokkaido Prefecture, Japan. Questionnaires were distributed to 1,245 teachers in public junior high schools. Information about basic characteristics, including working hours, and responses to the General Health Questionnaire-28 were collected anonymously. Multiple logistic regression analysis was used to calculate odds ratios (ORs) for the association between long working hours and psychological distress by gender. Results: Of the 1,245 teachers contacted, 558 (44.8%) responded. After excluding responses with missing data, the final sample included 522 teachers (337 males and 185 females). Psychological distress was identified in 47.8% of males and 57.8% of females. Our results showed a significantly increased risk only in males working >60 hours per week (adjusted OR=4.71 [95% CI 2.04–11.56]) compared with those working ≤40 hours per week. There were no significant associations between long working hours and psychological distress for females. Conclusions: There is a significant association between long working hours and psychological distress in male teachers. However, the causal relationship remains unclear. Further studies such as cohort studies with large sample sizes are needed.(J Occup Health 2015; 57: 20–27)