A heat pump apparatus has a main refrigerant circuit constituted by a closed loop including a compressor, condenser, gas-liquid separator, pressure reducer such as an expansion valve, and an evaporator. A shunting refrigerant line having a compressor, condenser and a pressure reducer means shunts from the gaseous phase portion of the gas-liquid separator of the main refrigerant circuit and merges in the evaporator of the main refrigerant circuit. The shunting refrigerant line may be constructed to have one or more compression stages.