A cross-sectional study was conducted to clarify the contribution of psychological job stress to insomnia in shift workers. A self-administered questionnaire concerning job stress, sleep, depressive symptoms and lifestyle factors was submitted to a sample of 530 rotating shift workers of age 18-59 years (mean age 27) in an electric equipment manufacturing company. Perceived job stress, i.e., job demands, job control and social support at work, was assessed using the Japanese version of the Job Content Questionnaire. Insomnia was regarded as prevalent if the workers had at least one of the following symptoms in the last year; less than 30 minutes to fall asleep, difficulty in maintaining sleep, or early morning awakening almost everyday. Overall prevalence was 37.8%. Logistic regression analyses while adjusting relevant factors showed that lower social support at work was significantly associated with a greater risk of insomnia than the higher social support (adjusted OR 2.5). Higher job strain with lower social support at work increased the risk, compared to lower strain with higher support at work (crude OR 1.8; adjusted OR 1.5). Our findings suggest the low social support at work independently associated with insomnia in shift workers.