An air conditioner incorporates valve devices mounted in a plurality of indoor units, respectively, for allowing refrigerant discharged from an outdoor heat exchanger to be introduced into one or more indoor units that are in operation. The refrigerant is prevented from being introduced into the other indoor units that are not in use, and therefore, oil is prevented from being accumulated in the other indoor units that are not in use. Also, an appropriate amount of refrigerant is supplied to the indoor units that are in operation, and therefore, reliability and efficiency of the air conditioner are improved.