The disaster at the Fukushima Daiichi Nuclear Power Plant after the Great East Japan Earthquake on March 11, 2011, might have affected the mental health status of children. To assess the mental health status, we measured the Strengths and Difficulties Questionnaire (SDQ) in 15 274 children (aged 4-15 years). The proportions of those who scored above the cutoff (>=16) of SDQ, reflecting the clinical range of the mental health status, were 25.0% (aged 4-6 years), 22.0% (aged 7-12 years, and 16.3% (aged 13-15 years), which were higher than that in the usual state (9.5%). We also explored the possibility that the distribution on the Fukushima prefectural map of the proportions of those who scored above the cutoff (>=16) of SDQ might correspond with the environmental radiation levels, but there was no significant correlation.