There is growing concern over the possible increase in mental health problems among Japanese workers. This trend is generally regarded as a reflection of Japan's prolonged economic depression and changes in working environment. In fact, claims for compensation for industrial accidents related to mental health diseases have been rapidly increasing in recent years. Working hours, personal-relationships, support from supervisors/co-workers, job demand, job control, and payment are known to affect workers mental health. In 2004, the Government announced a guideline to combat overwork and mental health problems at work places. This guideline articulates that long overtime working is a major indicator, and workers who work over 100 h overtime in a month should be encouraged to see an occupational physician. This guideline takes into account the practicalities of occupational health at work places and the empiric knowledge that long working hours might associate with workers mental health status. It may be reasonable to assume that long working hours affect workers health status both psychologically and physiologically, interacting with a variety of occupational factors, particularly job stress. However, the association between working hours and workers mental health status has not been fully clarified. The present article aimed to provide a systematic review of the association between working hours and mental health problems. The authors conducted a systematic review of the published literature on the association between working hours and mental health problems using PubMed. Of 131 abstracts and citations reviewed, 17 studies met the predefined criteria. Ten of these are longitudinal studies, and the others are cross-sectional studies. Seven of the 17 studies report statistically significant associations between working hours and mental health problems, while the others report no association. In addition, comparison among these studies is difficult because a variety of measurements of working hours were used. The present review found inconsistent results in the association between working hours and mental health burden.