This study aims at investigating the actual conditions of indoor environment in schools in order to obtain fundamental information for proper ventilation design for buildings. Indoor environment of two new elementary schools in Tohoku district of Japan was investigated for a week in the winter of 2005. Temperature and humidity, concentrations of carbon dioxide (CO 2), ventilation airflow rates, concentrations of chemical substances, and the opening condition of the windows and doors were measured and recorded in the three classrooms of each school. The CO 2 concentration was higher than 1500 ppm in many hours due to the lack of enough ventilation in classroom and corridor. Exhaust airflow rate was only 10 percent of the design values. This is due to no maintenance of the ventilation systems and no cleaning of filters. It was observed that meshes for protecting insect of the outlet of exhaust ducts. Maintenance of ventilation system is very important in order to increase exhaust airflow rate and improve indoor air quality. Fresh air was supplied 80 percent from the windows and doors facing corridor by passive method. But air supply from the corridor should be avoided if the air along corridor would be contaminated.