PURPOSE: To prevent to form a constant air flow path in a room and permit to re-utilize heat existing in discharging air by a method wherein suched air, whose amount is slightly larger than the amount of discharging air by ventilating effect, is introduced into the room.

CONSTITUTION: When the set temperature of a room 1 to be ventilated and atmospheric temperature is different under a condition that the atmosphere is to be introduced and heated or cooled to the set temperature, a heat exchanger 5 is provided in a discharging path, through which air in the room 1 is discharged forcibly into atmosphere, to absorb the amount of heat of air in the room, whose temperature is different from atmospheric temperature, the absorbed heat is utilized for the heat medium source of a refrigerating system 9 and the heat medium, whose heat is amplified by the refrigerating system 9, is introduced into the heat exchanger 4 installed at the atmosphere intake port 2 of the room 1 to be ventilated while heat exchange is effected between the heat medium and atmosphere, forcibly passed through the heat exchanger 4, thereafter, the atmospheric air is introduced into the room 1 to be ventilated. According to this method, latent heat of air, discharged out of the room 1 to be ventilated, may be re-utilized without wasting it.