To clarify the indoor climate in Japanese college classrooms, an air-conditioned, mechanically ventilated classroom of a university was surveyed. Temperatures, humidity and carbon dioxide (CO2) concentration in winter and summer were measured before, during and after lessons. The airtightness of the room and the airflow rate of the ventilation system were also measured. In winter, at an outdoor air temperature around 0 ºC and with the thermostat temperature of the air conditioners set to 30 ºC, the vertical difference in room air temperature exceeded 10 ºC. The relative humidity (RH) was about 30% during lesson hours. After half an hour of occupancy, the CO2 concentration exceeded 1500 ppm in the respiration zone. It subsequently reached 2500 ppm, as measured within the respiration zone at half room height, after a continued 1.5 hours of occupancy. No special thermal or humidity problems were found in the summer survey. However, the CO2 level was very close to that of winter. The CO2 concentration decays indicated that the air change rate was 0.82 to 0.87 ac/h, which satisfied the Japanese criterion of 0.3 ac/h for the prevention of formaldehyde, yet it was far below the national criterion of 4.4 ac/h for college classrooms.