In an air conditioning system, a damper unit disposed in the middle of a circulating duct is controlled so that a throttle valve of at least one of air quantity regulating devices for controlling incoming outside air quantity and exhaust air quantity is fully open, and that the quantity of air passing through the device whose throttle valve is fully open is equal to a set air quantity. The control system for the damper unit is so-called floating control system that is responsive to the control conditions. If none of the throttle valves of the devices is fully open, then there must be too great a pressure difference to pass the set quantity of air through all the devices. Accordingly, the excessive pressure applied to the devices can be lowered to a proper level by driving the damper unit in an opening direction, thereby reducing pressure loss between the discharging and charging blowers. If the throttle valve of at least one of the devices is fully open, and if the passage air quantity is smaller than the set air quantity, then the pressure applied to the devices must be too small to pass the set quantity of air. Accordingly, the insufficient pressure applied to the devices can be increased to the proper level by driving the damper unit in a closing direction, thereby increasing the resistance between the discharging and charging blowers.