This study estimates the effects of fathers’ hours worked on their children’s behavior. To identify the source of the exogenous change, it uses the legislated reduction in the work week that was implemented in Korea. Since the new system was applied to large establishments first, one could analyze the effects using a difference-in-differences estimation method. This study finds that the system change increases boys’ hours of self-study and decreases their alcohol use (reduced-form estimates). The channel is fathers’ increased knowledge of their children’s whereabouts and the children’s decreased conflict with their parents. However, no effects are found on girls’ behavior.