Objectives: As the number of dual-earner couples in Japan has increased, work-life balance has become important. This study aimed to examine the factors that contribute to work-family conflict. Methods: The participants included 3,594 (2,332 men and 1,262 women) civil servants aged 20–59 working for local government on the west coast of Japan. Logistic regression analysis was used to evaluate whether work, family, or lifestyle characteristics were associated with work-family conflict. Results: For men, family-to-work conflict was associated with being elderly, having low-grade employment, working long hours, raising children, and sleeping shorter hours. For women, being married and raising children were strong determinants of family-to-work conflict, and being middle-aged, working long hours, and sleeping shorter hours were also associated with this type of conflict. Regarding work-to-family conflict, working long hours was the strongest determinant of conflict in both sexes. In men, being elderly, living with family, eating dinner late, and sleeping shorter hours were also associated with work-to-family conflict. In women, having high-grade employment, being married, raising children, and eating dinner late were associated with work-to-family conflict. Conclusions: This study showed that working long hours was the primary determinant of work-to-family conflict in both sexes and that being married and raising children were strong factors of family-to-work conflict in women only. Sex differences may reflect divergence of the social and domestic roles of men and women in Japanese society. To improve the work-life balance, general and sex-specific health policies may be required.(J Occup Health 2014; 56: 485–497)