Since the 1970s Japanese families are in a process of transformation – their size, housing arrangements, and lifestyles. Yet married couples with children continue to constitute the dominant form of families in Japan. While this “traditional” family model is still valid, the social environment and the economy have been going through significant changes, triggering surged public attention to people’s well-being. Against this background, this article studies the sexual division of household labor, a major feature of the postwar Japanese family system, and its relation to well-being. Data come from a nation-wide survey among 2,000 Japanese mothers and fathers of young children up to six years old. Well-being is measured in 16 separate areas on 11-point Likert satisfaction scales, with focus on the differences between mothers’ and fathers’ well-being. The sexual division of household labor is measured in actual and ideal household share contribution. We found a significant gender gap in household labor input between husbands and wives and in their satisfaction levels. Employment and working hours were found to have partial effect on husbands’, but almost no effect on wives’ mean satisfaction scores. We argue that despite all the external changes surrounding Japanese families, such as mothers’ increased labor activities, the domestic sphere has remained highly gendered and is a source of dissatisfaction of mothers relative to those of fathers.