To clarify the relationship between perceived job stress, social support and prevalence of insomnia in Japanese daytime workers, 1161 male white-collar employees of an electric equipment manufacturing company (age, 23-60 years, mean age of 37.0) were surveyed by means of a mailed questionnaire. Perceived job stress was evaluated with the Japanese version of the generic NIOSH job stress questionnaire. Insomnia was diagnosed if workers had at least 1 of 3 types of symptoms on an almost nightly basis. The symptoms were (1) taking more than 30 min to fall asleep (Difficulty Initiating Sleep, DIS), (2) difficulty in maintaining sleep (DMS), or (3) early morning awakening (EMA). The overall prevalence of insomnia was 23.6% and the prevalences of the three subtypes were 11.3% for DIS, 14.2% for DMS, and 1.9% for EMA. Workers with high intragroup conflict (OR 1.6), high job dissatisfaction (OR 1.5), and high symptoms of depression (OR 2.0) (CES-D > 16) had a significantly increased risk for insomnia after adjusting for multiple confounding factors. Low employment opportunities, physical environment and low coworker support also were weakly associated with risk for insomnia among workers. Furthermore, high depressive symptoms significantly increased the risk of DIS (OR 2.4). Therefore in white-collar male daytime workers, psychological job stress factors such as interpersonal conflicts with fellow employees, job satisfaction, and social support were independently associated with a modestly increased risk of insomnia that included three different subtypes that were considered to be defining for the disorder.