An air conditioning system for a vehicle has a first cycle including a compressor, a first condenser, a second condenser, a liquid tank, an expansion valve and an evaporator which are connected through pipes to form a first refrigerant circulation circuit through which a refrigerant flows from an outlet of the compressor to an inlet of the compressor while changing the phase. The first cycle causes the evaporator to act as a cooler when operated. The system further has a second cycle including the compressor, the second condenser, the liquid tank, the evaporator and the evaporator which are connected through pipes to form a second refrigerant circulation circuit through which the refrigerant flows from the outlet of the compressor to the inlet of the compressor while changing the phase. The second cycle causes the condenser to act as a heater and the evaporator to act as a cooler when operated. The first and second cycles are switched by switch means. The system further has an air duct case having an air flow passage in which the second condenser and the evaporator are installed. The air flow passage is connected at its downstream part to a passenger room of the vehicle. The air conditioning system further comprises heater means for heating a returning refrigerant which, under operation of the second cycle, flows in the pipe extending from an outlet of the evaporator to the inlet of the compressor.