Objectives: The purpose of this study was to investigate the cross-sectional association of employment contract, company size, and occupation with psychological distress using a nationally representative sample of the Japanese population. Methods: From June through July 2007, a total of 9,461 male and 7,717 female employees living in the community were randomly selected and surveyed using a self-administered questionnaire and interview including questions about occupational class variables, psychological distress (K6 scale), treatment for mental disorders, and other covariates. Results: Among males, part-time workers had a significantly higher prevalence of psychological distress than permanent workers. Among females, temporary/contract workers had a significantly higher prevalence of psychological distress than permanent workers. Among males, those who worked at companies with 300-999 employees had a significantly higher prevalence of psychological distress than those who worked at the smallest companies (with 1-29 employees). Company size was not significantly associated with psychological distress among females. Additionally, occupation was not significantly associated with psychological distress among males or females. Similar patterns were observed when the analyses were conducted for those who had psychological distress and/or received treatment for mental disorders. Conclusions: Working as part-time workers, for males, and as temporary/contract workers, for females, may be associated with poor mental health in Japan. No clear gradient in mental health along company size or occupation was observed in Japan.