During the early days of the first outbreak of the novel coronavirus disease 2019 (COVID-19) in Japan, preschools were exempted from the nationwide school closure, which came into effect on March 2, 2020. However, the decision to close or open preschools was left to each facility and the municipality of their location, and the decision was characterized by high randomness. In addition to this, Japan is the rare country that experienced preschool closure without any heavy restrictions on daily life activities, which makes it possible to separate the effects of preschool closure from the effects of other anti-COVID-19 policies such as lockdown. Utilizing these advantages, we explored the effects of children's absence from preschool during March 2020 on mothers' psychological distress. Our results show that preschool closure caused an immediate deterioration in mothers' psychological state. The effect of children's absence from preschool on moderate psychological distress persisted for at least five months.