Frequencies and characteristics of heatstroke occurred at elementary/secondary schools were investigated by using the data of NAASH (National Agency for the Advancement of Sports and Health). The environmental factors, e.g., outdoor temperature, relative humidity and global solar radiation, were obtained at the closest meteorological observatory at the time of occurrence. 1328 cases of heatstroke were reported in the data of NAASH for the investigated schools. Of the 1328 cases, 836 were occurred at school area, i.e., schoolyard and school building. Of the 836 cases occurred at school, 533 were occurred on schoolyard, 204 were in gymnasium, 66 were in school house, and 20 were in swimming pool. Out of the 836 cases of heatstroke at school, 503 cases were occurred during athletic extracurricular activities, 104 were occurred on field day, 85 were occurred during the class of physical education. The relationships between the environmental factors, i.e., outdoor temperature, wet-bulb temperature, WBGT (wet-bulb globe temperature), and humidity ratio, and the risk factor, i.e., the odds ratio of heatstroke, were obtained. The risk factors of heatstroke for schoolyard, gymnasium, and school house, had a good correlation with WBGT, and this relationship would be used for the evidence when designing safe school environment.