OBJECTIVE: To examine the association between the amount of time spent at childcare and diet quality in 668 Japanese children aged 1·5-6 years. DESIGN: A cross-sectional design was used. Dietary information was collected using dietary records (1 d for children aged 1·5-2 years and 2 d for children aged 3-6 years). Diet quality was assessed by counting the number of nutrients not meeting the Japanese Dietary Reference Intakes (DRI). Each child's guardian reported the average amount of time spent at childcare per d for the previous 1 month. SETTING: In total, 315 childcare centres located in twenty-four areas in Japan. PARTICIPANTS: In total, 753 children aged 1·5-6 years who attend childcare facilities. RESULTS: After adjustment for potential confounders, OR for the low diet quality (≥ 5 of twenty nutrients not meeting DRI) in long (≥10 h/d) v. medium (8-10 h/d) childcare hours was 4·81 (95 % CI 1·96, 11·8) among children aged 5-6 years. There was no significant association in children aged 1·5-2 and 3-4 years. CONCLUSION: This study showed that long time spent at childcare was strongly associated with low diet quality among children aged 5-6 years, but not those aged 1·5-2 and 3-4 years. More research is needed to clarify different associations in each age group.