A heat pump apparatus that having a refrigerating cycle including a compressor, a gas cooler, a pressure reducing device and an evaporator and is designed so that water can be heated by the gas cooler. A refrigerant such as CO₂ or the like working in a supercritical area is filled and used at a high-pressure side in the refrigerating cycle and a radiator is equipped between the exit of the gas cooler and the pressure reducing device. The refrigerant temperature at the exit of the radiator is kept at a substantially fixed temperature irrespective of the temperature of hot water to the gas cooler.