A heat pump system (1, 200, 300) comprising: a heat-source-side refrigerant circuit having a variable-capacity heat-source-side compressor for compressing heat-source-side refrigerant, a first usage-side heat exchanger (41a, 41b) capable of functioning as a radiator of the heat-source-side refrigerant, a heat-source-side heat exchanger capable of functioning as an evaporator of the heat-source-side refrigerant, and a heat-source-side discharge pressure sensor for detecting a heat-source-side discharge pressure of the heat-source-side refrigerant in the discharge of the heat-source-side compressor; a usage-side refrigerant circuit (40a, 40b) having a variable-capacity usage-side compressor (62a, 62b) for compressing usage-side refrigerant, a refrigerant/water heat exchanger (65a, 65b) capable of functioning as a radiator of the usage-side refrigerant to heat an aqueous medium, the first usage-side heat exchanger (41a, 41b) capable of functioning as an evaporator of the usage-side refrigerant by radiation of the heat-source-side refrigerant, a usage-side discharge pressure sensor for detecting the usage-side discharge pressure of the usage-side refrigerant in the discharge of the usage-side compressor (62a, 62b), and a usage-side discharge temperature sensor for detecting the usage-side discharge temperature of the usage-side refrigerant in the discharge of the usage-side compressor (62a, 62b); and wherein the heat pump system comprises a control unit which is configured to control the capacity of the heat-source-side compressor so that a heat-source-side discharge saturation temperature, which is the saturation temperature corresponding to the pressure of the heat-source-side refrigerant in the discharge of the heat-source-side compressor, becomes a predetermined target heat-source-side discharge saturation temperature; and to control the capacity of the usage-side compressor (62a, 62b) so that a usage-side discharge saturation temperature, which is the saturation temperature corresponding to the pressure of the usage-side refrigerant in the discharge of the usage-side compressor, becomes a predetermined target usage-side discharge saturation temperature. a heat-source-side refrigerant circuit having a variable-capacity heat-source-side compressor for compressing heat-source-side refrigerant, a first usage-side heat exchanger (41a, 41b) capable of functioning as a radiator of the heat-source-side refrigerant, a heat-source-side heat exchanger capable of functioning as an evaporator of the heat-source-side refrigerant, and a heat-source-side discharge pressure sensor for detecting a heat-source-side discharge pressure of the heat-source-side refrigerant in the discharge of the heat-source-side compressor; a usage-side refrigerant circuit (40a, 40b) having a variable-capacity usage-side compressor (62a, 62b) for compressing usage-side refrigerant, a refrigerant/water heat exchanger (65a, 65b) capable of functioning as a radiator of the usage-side refrigerant to heat an aqueous medium, the first usage-side heat exchanger (41a, 41b) capable of functioning as an evaporator of the usage-side refrigerant by radiation of the heat-source-side refrigerant, a usage-side discharge pressure sensor for detecting the usage-side discharge pressure of the usage-side refrigerant in the discharge of the usage-side compressor (62a, 62b), and a usage-side discharge temperature sensor for detecting the usage-side discharge temperature of the usage-side refrigerant in the discharge of the usage-side compressor (62a, 62b); and wherein the heat pump system comprises a control unit which is configured to control the capacity of the heat-source-side compressor so that a heat-source-side discharge saturation temperature, which is the saturation temperature corresponding to the pressure of the heat-source-side refrigerant in the discharge of the heat-source-side compressor, becomes a predetermined target heat-source-side discharge saturation temperature; and to control the capacity of the usage-side compressor (62a, 62b) so that a usage-side discharge saturation temperature, which is the saturation temperature corresponding to the pressure of the usage-side refrigerant in the discharge of the usage-side compressor, becomes a predetermined target usage-side discharge saturation temperature.