The air quality during winter period in elementary/secondary schools located in metropolitan district X was analyzed by using results of the environmental hygienic audit. The period of analysis was set as 3 years from 2007 to 2009. The following conclusions were obtained; 1) The average values of air temperature, the relative humidity, and CO2 concentration in classrooms of the elementary schools met the standard of school environmental hygiene. 2) The ratio of the CO2 observations exceeding the standard value of 1500 ppm in the elementary schools was 30.5 %, and that in the secondary schools was 58.5 %. 3) The clear tendency that the lower the outdoor temperature, the higher the heating usage ratio, was observed both in the elementary and the secondary schools. Under the same outdoor temperature condition, the heating usage ratio in the secondary schools was higher than that in the elementary schools. 4) Under the same outdoor temperature condition, the average room temperature in the classrooms of the secondary schools was significantly higher than that of the elementary schools. The average CO2 concentrations in the secondary schools exceeded the standard values under the all temperature classes.