Abstract: A heat pump system usable as an air-conditioning apparatus, a refrigeration apparatus, or a temperature-conditioning apparatus, having stable cooling, heating, or freezing capacity without significant influence from the difference in height between the expansion valve and the compressor, and the length of piping of the refrigerant circulation line, which system comprises a pressure sensing means for measuring the pressure in the refrigerant line immediately upstream of the expansion valve; optionally a pressure sensing means for measuring the pressure in the refrigerant line immediately downstream of the compressor; and an energy-supplying mechanism for exerting energy onto the refrigerant when the measured pressure is lower than a predetermined value, thereby controlling the pressure in the high pressure refrigerant line.