PURPOSE: To positively hold the balance of air quantities of ducts, to maintain indoor environments and to provide a stability of control in the concentration of a carbonic gas and the like by controlling the air quantities by signals from detectors so that respective varied quantities of air passing through an external air take-in duct, an exhaust duct and a ventilation duct become equal to each other.

CONSTITUTION: A signal responsive to the carbonic gas concentration is generated by a carbonic gas concentration detector 18 disposed in a ventilation duct 14, and inputted into an air quantity indicator 19. The carbonic gas concentration desired to be controlled is preset in the air quantity indicator 19, which sends an indication signal to a variable constant air quantity device responsive to a difference between the preset signal and the signal from the carbonic gas concentration detector. The maximum processed air quantities of the variable constant air quantity devices 15, 16 and 17 are determined beforehand. By the above signal difference, the air quantity control of the variable constant air quantity device 15 for taking in external air, is carried out, and external air is sent to the air conditioner 13. In the variable constant air quantity device 16 for air exhaustion, an air quantity obtained by subtracting independent air exhaustion quantities such as those of toilet and the like from the external air taken in quantity, is exhausted outside. The variable constant air quantity device for ventilation 17 is controlled so that it returns an air quantity subtracting the exhaustion quantity from the total ventilation quantity to the air conditioner 13.