Background: Most Japanese daycare centers do not accept ill children, even if they only have minor symptoms. Nurseries for children with mild illness have been developed (mostly in urban areas), but use of these nurseries dropped sharply during the COVID-19 pandemic. There are two possible reasons for this decrease: efforts to prevent COVID-19 infection reduced common infectious diseases in children and eliminated the need for these nurseries, or parents refrained from using these nurseries because they feared a COVID-19 outbreak. This study aimed to clarify the reason for the decreased use of these nurseries. Methods: Data on trends in common infectious diseases reported by sentinel medical facilities to public health centers in Hiroshima Prefecture, Japan from January 2018 to March 2021 were provided by the Hiroshima CDC (Center for Disease Control and Prevention). The number of monthly users of nurseries for children with mild illness in the 23 municipalities in Hiroshima Prefecture over the same period was obtained by a questionnaire survey. Multiple regression analysis was used to assess the association between the numbers of nursery users and common infectious diseases before and during the COVID-19 pandemic.Results: The regression coefficient between the number of users of these nurseries and number of common infectious diseases did not change between before and during the COVID-19 pandemic, except for an increase in intercept during the pandemic.Conclusions: The decreased number of children with mild illnesses using nurseries during the COVID-19 pandemic was related to the decrease in common infectious diseases, and not to parents refraining from using nurseries because they feared a COVID-19 outbreak. Many parents may still have wished to use nurseries for children with mild illness during the COVID-19 pandemic.