Windows and doors of the classrooms were kept open throughout a year against COVID-19 at elementary school without ventilation equipment. The estimated ventilation during heating period was less obtained than that during the other period. It is thought that the opening width of the window in winter was regulated to reduce the influence of the chill. However, pupils who were in sedentary activity, were exposed to ambient air temperature lower than 18 ℃. The estimated ventilation rate per person more than 11m 3 /h was obtained in the class with Hybrid-Flexible lesson in winter.