AbstractBackgroundChild poverty is a growing, serious issue in Japan, where various social disparities are increasing. Numerous reports have focused on the relationship between social inequity and health, but few studies have assessed how the overall magnitude of disparities in child health has changed in the course of drastic social and economic transitions from 1899 to more recent times. In this study, we assessed the trend of the under‐5 mortality rate (U5MR) and its inter‐prefecture disparity in Japan.MethodsThis is a secondary analysis of Japan's vital statistics data from 1899 to 2014 (115 years), which covers a core period of modern Japan. We calculated the U5MR of each prefecture and its Theil index by year to assess the trend of inter‐prefecture disparity in child health from 1899 to 2014.ResultsThe U5MR monotonically decreased from 238 per 1,000 births in 1899 to 3 in 2014. The Theil index of the U5MR increased in the post‐war period, peaked in 1962 (0.027) and gradually reduced to <0.01 in the 1970s. In the 2000s, however, even though U5MR continued to decrease, the Theil index started to increase, and in 2014 (0.013) it exceeded that in 1970 and was more similar to that before World War II.ConclusionsThe disparities in child health appear to be widening, and may serve as a warning to today's society that increasing socioeconomic gradients may lead to rising health inequity among children. Further investigations into the causes, mechanisms, and possible interventions are needed.