PURPOSE: To enable an improvement of indoor environment to be attained with a low cost by a method wherein an amount of dust in an indoor area is detected and if this detected amount is larger than its predetermined value a control over over outer air taking operation based on the detected amount is preferentially performed.

CONSTITUTION: A dust sensor 9 detects an amount of dust in an indoor area and outputs a degree-of-opening signal A for use in controlling a degree-of- opening of a surrounding air bypass damper 5 and a degree-of-opening of a discharging bypassing damper 6. Irrespective of available or non-available outer air cooling, if the detected value of an amount of dust is higher than its predetermined value, a full heat exchanger 2 is turned off and then a degree-of-opening of the damper 5 and a degree-of-opening of the damper 6 are controlled by a signal A, resulting in that a discharging of the indoor air is promoted and an amount of dust is decreased. That is, a reduction of an amount of dust is preferentially controlled rather than an indoor air temperature.