To operate a total heat exchanger flexibly in accordance with the environment in a room, and to achieve energy saving while securing optimum air conditioning.

An equivalent air amount corresponding to an amount of the air and the total heat replaced by the total heat exchanger per unit time is adopted in controlling of the total heat exchanger. The outside air cooling mode same with the present mode is selected when the temperature in the room is within a comfortable range. The outside cooling mode in accordance with the large equivalent air amount is selected when the temperature in the room is higher than an upper limit value of the comfortable range. The outside cooling mode in accordance with the small equivalent air amount is selected when the temperature in the room is lower than a lower limit value of the comfortable range (S6). The amount of the air and the total heat replaced by the total heat exchanger per unit time is controlled based on the selected outside air cooling mode (S7).