Studies across Europe and the US report that childhood socioeconomic disadvantage is associated with poorer health in adulthood. By contrast, a study in Japan suggests that childhood socioeconomic disadvantage may be positive for adult health. In this paper, we assess the association between intergenerational income mobility and self-rated health in Japan, using detailed childhood income data for 1610 men and 1885 women aged 30-49 years. We use an instrumental variable approach to identify the causal effect of upward income mobility on adult health. We find that low father's income during childhood is associated with smoking and alcohol consumption in adult life for both men and women. For men, upward income mobility was associated with worse health. Certain behavioural choices related to income mobility, such as long working hours, may have detrimental health effects.