A heat pump vehicle air conditioning system passes hot gas to a condenser and a vaporizer for the vehicle cabin exterior. Even with low outside air temperature, it can efficiently defrost using the cooling cycle. The system includes a cabin-interior condenser downstream of a cabin-interior vaporizer within an HVAC unit connected via refrigerant switching means to a cooling cycle. Cooling is accomplished by a compressor, cabin-exterior condenser, first decompression unit with on-off valve function, the cabin-interior vaporizer, and an accumulator. A cabin-exterior vaporizer disposed outside of the vehicle cabin is connected with a second decompression unit via an on-off valve function and to the exit-side liquid refrigerant pipe of the cabin-exterior condenser. A bypass circuit having a third decompression unit with on-off function is connected between the accumulator and the exit-side liquid refrigerant pipe of the cabin-exterior condenser.