This cross-sectional study examined the relationship between temperament, job stress, and overcommitment using the Temperament Evaluation of Memphis, Pisa, Paris and San Diego-Autoquestionnaire version (TEMPS-A) and a scale of the effort-reward imbalance (ERI) model. In July 2004, self-administered questionnaires were distributed to all employees in a Japanese IT service company through the company postal system. Total response rate was 63% (N=874), with 730 completed questionnaires. Information collected included individual attributes, employment and organizational characteristics. The TEMPS-A and the Japanese version of the ERI questionnaire were self-administered. The completed data of 637 personal computer technical support staff (87%) were used in a hierarchical regression analysis. Our results showed that depressive and anxious temperaments attenuate the influence of working hours and influence effort and rewards independently. While actual working hours had more impact on perceived high effort, our findings regarding rewards suggest that understanding anxious and depressive temperaments has a significant role in stress self-management. Temperaments explained 36% of the variance of overcommitment, and the variance was more than that of mean working hours. Our research has provided meaningful insights into occupational health, which could assist employees in self-management of job stress and contribute to better adaptation at the workplace.