In Japan, schools were suddenly closed all over the country with one month left at the end of the school year to prevent the spread of new coronavirus infections. Although previous studies have shown that quarantine and isolation have a negative impact on mental health, the impact of school closure on children's mental health is not clear. This study examined mental health indicators, such as emotional symptoms, in addition to behavior during school closure and anxiety after school resumes, among children in three primary schools for which survey data existed in December before school closure. No cases of infection have been confirmed in the area where the school was located at the time of the survey. As a result, anxiety about whether they would be able to return to their normal routine after school resumed was associated with mental health. In addition, there was a worsening of emotional symptoms in those who were in medium group of school adjustment as of December.