To provide a new heat pump, and a new radiator used in the same.

This heat pump uses carbon dioxide as a refrigerant, and includes a compressing process, a radiating process, an expanding process and a heating process. In the expanding process, a turbine 8 recovering expanding work is disposed, and a maximum pressure of the refrigerant is set within a range of 7.5-9.5 MPa. Here, the type of the turbine 8 is an impulse turbine or a reaction turbine, and the electric power generated by the power of the turbine 8 is preferably supplied as the power of a compressor 1 used in the compressing process. Further a low-temperature medium in the radiating process is the water, a high-temperature medium in the heating process is the air, and the heat pump is preferably used as a water heater. The radiator 2 constituting the radiating process is composed of PCHE (Printed Circuit Heat Exchanger), and PCHE is formed by diffusion bonding of a plurality of plates provided with flow channels.