The spread of the novel coronavirus disease caused schools in Japan to close to cope with the pandemic. In response to this, parents of students were obliged to care for their children during the daytime when they were usually at school. Does the increase in burden of childcare influence parents mental health? Based on short panel data from mid-March to mid-April 2020, we explored how school closures influenced the mental health of parents with school-aged children. Using the fixed effects model, we found that school closures lead to students mothers suffering from worse mental health than other females, while the fathers mental health did not differ from other males. This tendency was only observed for less educated mothers who had children attending primary school, but not those attending junior high school. The contribution of this paper is to show that school closures increased the inequality of mental health between genders and the educational background of parents.