A heat pump system (1, 200, 300) comprising: a heat-source-side refrigerant circuit which has a heat-source-side compressor for compressing a heat-source-side refrigerant, a first usage-side heat exchanger (41a, 41b) capable of functioning as a radiator of the heat-source-side refrigerant, and a heat- source-side heat exchanger capable of functioning as an evaporator of the heat-source- side refrigerant; and a usage-side refrigerant circuit (40a, 40b) which has a usage-side compressor (62a, 62b) for compressing a usage-side refrigerant whose pressure corresponding to a saturated gas temperature of 65°C is 2.8 MPa or less at gauge pressure, a refrigerant-water heat exchanger (65a, 65b) capable of functioning as a radiator of the usage-side refrigerant and heating an aqueous medium, and the first usage-side heat exchanger (41a, 41b) capable of functioning as an evaporator of the usage-side refrigerant by the radiation of the heat-source- side refrigerant; wherein the usage-side compressor, the first usage-side heat exchanger, and the refrigerant-water heat exchanger constitute a first usage unit (4a, 4b); and the usage-side refrigerant circuit is not provided with an oil separating mechanism for separating refrigeration, machine oil contained in the usage-side refrigerant discharged from the usage-side compressor and returning the oil to the intake of the usage-side compressor; characterized in that: the length of a refrigerant tube from the first usage-side heat exchanger functioning as an evaporator of the usage-side refrigerant to the usage-side compressor is 3 m or less; and the weight of the usage-side refrigerant enclosed in the usage-side refrigerant circuit is one to three times the weight of the refrigeration machine oil enclosed for lubricating the usage-side compressor. a heat-source-side refrigerant circuit which has a heat-source-side compressor for compressing a heat-source-side refrigerant, a first usage-side heat exchanger (41a, 41b) capable of functioning as a radiator of the heat-source-side refrigerant, and a heat- source-side heat exchanger capable of functioning as an evaporator of the heat-source- side refrigerant; and a usage-side refrigerant circuit (40a, 40b) which has a usage-side compressor (62a, 62b) for compressing a usage-side refrigerant whose pressure corresponding to a saturated gas temperature of 65°C is 2.8 MPa or less at gauge pressure, a refrigerant-water heat exchanger (65a, 65b) capable of functioning as a radiator of the usage-side refrigerant and heating an aqueous medium, and the first usage-side heat exchanger (41a, 41b) capable of functioning as an evaporator of the usage-side refrigerant by the radiation of the heat-source- side refrigerant; wherein the usage-side compressor, the first usage-side heat exchanger, and the refrigerant-water heat exchanger constitute a first usage unit (4a, 4b); and the usage-side refrigerant circuit is not provided with an oil separating mechanism for separating refrigeration, machine oil contained in the usage-side refrigerant discharged from the usage-side compressor and returning the oil to the intake of the usage-side compressor; characterized in that: the length of a refrigerant tube from the first usage-side heat exchanger functioning as an evaporator of the usage-side refrigerant to the usage-side compressor is 3 m or less; and the weight of the usage-side refrigerant enclosed in the usage-side refrigerant circuit is one to three times the weight of the refrigeration machine oil enclosed for lubricating the usage-side compressor. the length of a refrigerant tube from the first usage-side heat exchanger functioning as an evaporator of the usage-side refrigerant to the usage-side compressor is 3 m or less; and the weight of the usage-side refrigerant enclosed in the usage-side refrigerant circuit is one to three times the weight of the refrigeration machine oil enclosed for lubricating the usage-side compressor.