An air conditioner for vehicles includes a refrigerant circuit having a compressor capable of varying its output, a heat sink and a radiator disposed in an air duct for cooling operation, heating operation and dehumidifying operation modes; sensors for detecting air temperatures at the output-side positions of the heat sink and the radiator, respectively; and a control unit for controlling the output of the compressor in response to at least one of the detected air temperatures. The heat absorbing action due to the heat sink and the heat radiating action due to the radiator are appropriately controlled in the refrigerant circuit side in the respective operation modes without unnecessary heat absorbing and heat radiating actions. Further, the temperature of the air discharged from the air duct into the interior of the vehicle can be properly controlled by adjustment of the heat absorbing ability of the heat sink and the heat radiating ability of the radiator by the control of the compressor output.