A heat pump system comprising: a heat source unit having a variable-capacity compressor and a heat-source-side heat exchanger functioning as an evaporator for a refrigerant; and a plurality of usage units (4a, 4b, 10a) connected to the heat source unit, and having a usage-side heat exchanger (41a, 41b, 101a) that functions as a radiator for the refrigerant; wherein the heat pump system is configured to control an operating capacity of the compressor to bring the discharge pressure of the compressor, or a state quantity equivalent thereto, to a first target value; and the heat pump system is configured to determine the first target value on the basis of an equivalent target value equivalent to a usage temperature required in individual usage units (4a, 4b, 10a), characterized in that: the plurality of usage units (4a, 4b) includes a plurality of first usage units (4a, 4b) capable of heating an aqueous medium through radiation by the refrigerant in the individual usage-side heat exchangers (41a, 41b); the equivalent target value is a first equivalent target value based on an aqueous medium temperature equivalent to a usage temperature required in the individual first usage units (4a, 4b); the heat pump system is configured to determine the first target value on the basis of the greatest value among the equivalent target values of individual first usage units (4a, 4b); and the heat pump system is configured such that the first usage unit (4a, 4b) for which the first target value is greater than the first equivalent target value carries out a control of refrigerant flow rate control means (42a, 42b), using a correction value based on a differential of the first target value and the first equivalent target value. a heat source unit having a variable-capacity compressor and a heat-source-side heat exchanger functioning as an evaporator for a refrigerant; and a plurality of usage units (4a, 4b, 10a) connected to the heat source unit, and having a usage-side heat exchanger (41a, 41b, 101a) that functions as a radiator for the refrigerant; wherein the heat pump system is configured to control an operating capacity of the compressor to bring the discharge pressure of the compressor, or a state quantity equivalent thereto, to a first target value; and the heat pump system is configured to determine the first target value on the basis of an equivalent target value equivalent to a usage temperature required in individual usage units (4a, 4b, 10a), characterized in that: the plurality of usage units (4a, 4b) includes a plurality of first usage units (4a, 4b) capable of heating an aqueous medium through radiation by the refrigerant in the individual usage-side heat exchangers (41a, 41b); the equivalent target value is a first equivalent target value based on an aqueous medium temperature equivalent to a usage temperature required in the individual first usage units (4a, 4b); the heat pump system is configured to determine the first target value on the basis of the greatest value among the equivalent target values of individual first usage units (4a, 4b); and the heat pump system is configured such that the first usage unit (4a, 4b) for which the first target value is greater than the first equivalent target value carries out a control of refrigerant flow rate control means (42a, 42b), using a correction value based on a differential of the first target value and the first equivalent target value. the plurality of usage units (4a, 4b) includes a plurality of first usage units (4a, 4b) capable of heating an aqueous medium through radiation by the refrigerant in the individual usage-side heat exchangers (41a, 41b); the equivalent target value is a first equivalent target value based on an aqueous medium temperature equivalent to a usage temperature required in the individual first usage units (4a, 4b); the heat pump system is configured to determine the first target value on the basis of the greatest value among the equivalent target values of individual first usage units (4a, 4b); and the heat pump system is configured such that the first usage unit (4a, 4b) for which the first target value is greater than the first equivalent target value carries out a control of refrigerant flow rate control means (42a, 42b), using a correction value based on a differential of the first target value and the first equivalent target value.