This study aims at investigating the actual conditions of indoor environment in schools in order to obtainfundamental information for proper ventilation design for buildings. Indoor environment of two newelementary schools in Tohoku district of Japan was investigated for a week in the winter of 2005.Temperature and humidity, concentrations of carbon dioxide (CO2), ventilation airflow rates,concentrations of chemical substances, and the opening condition of the windows and doors weremeasured and recorded in the three classrooms of each school. The CO2 concentration was higherthan 1500 ppm in many hours due to the lack of enough ventilation in classroom and corridor. Exhaustairflow rate was only 10 percent of the design values. This is due to no maintenance of the ventilationsystems and no cleaning of filters. It was observed that meshes for protecting insect of the outlet ofexhaust ducts. Maintenance of ventilation system is very important in order to increase exhaust airflowrate and improve indoor air quality. Fresh air was supplied 80 percent from the windows and doorsfacing corridor by passive method. But air supply from the corridor should be avoided if the air alongcorridor would be contaminated.