The winter thermal environment and indoor air quality in classrooms has been reported to be very poor in Japan. In this study, an air-conditioned, mechanically ventilated classroom was surveyed. Air temperature, globe temperature and the concentration of CO2 were monitored before, during and after the three-hour occupancy by 35 adults. Airtightness and airflow rates of the ventilation system were also measured. With the outdoor air of about 0ºC and the thermostat temperature setting of 30ºC, the vertical difference in air temperature exceeded 10ºC, while the horizontal difference was under 2.5ºC. After half an hour of occupancy, CO2 concentration exceeded 1500 ppm in the respiration zone, and it reached 2500 ppm from the respiration zone to the middle of the room height for a consecutive 1.5-hour. The concentration decay showed that the air change rate was 0.82 ach. It was far from the national criterion of 2.2 ach.