A heat pump system, comprising: a refrigerant circuit having a compressor configured to compress a refrigerant, a heat source-side heat exchanger configured to function as an evaporator for the refrigerant, and a refrigerant-water heat exchanger configured to function as a heat radiator for the refrigerant and heat an aqueous medium; an aqueous medium circuit having a circulation pump and the refrigerant-water heat exchanger, in which the aqueous medium having exchanged heat with the refrigerant in the refrigerant-water heat exchanger circulates, and the aqueous medium circuit being connected to aqueous medium devices (8, 9) configured to use the aqueous medium to perform an operation; an auxiliary heater configured to further heat the aqueous medium circulating in the aqueous medium circuit, the auxiliary heater being provided to an aqueous medium outlet side of the refrigerant-water heat exchanger in the aqueous medium circuit; a heating capability computation unit configured to compute a heating capability of the aqueous medium devices (8, 9) on the basis of an operating state quantity of constituent devices or of the refrigerant flowing through the refrigerant circuit; a circulation flow rate computation unit configured to compute a circulation flow rate of the aqueous medium in the aqueous medium circuit on the basis of the heating capability and of an outlet/inlet temperature difference, which is the difference between an inlet temperature and outlet temperature of the aqueous medium in the refrigerant-water heat exchanger; and a prediction unit configured to predict an outlet temperature of the aqueous medium in the auxiliary heater in a case where the auxiliary heater has acted, on the basis of the circulation flow rate and of heat source capability information indicative of a capacity of the auxiliary heater, wherein the compressor is a variable-capacity-type compressor, and the heat pump system further comprises: a heat source operation controller configured to cause the auxiliary heater to perform an operation in a case where the capacity of the compressor is a predetermined capacity or higher, and an outlet-side temperature difference, which is the difference between a target outlet temperature and the outlet temperature of the aqueous medium in the refrigerant-water heat exchanger, is a first predetermined temperature difference or higher. a refrigerant circuit having a compressor configured to compress a refrigerant, a heat source-side heat exchanger configured to function as an evaporator for the refrigerant, and a refrigerant-water heat exchanger configured to function as a heat radiator for the refrigerant and heat an aqueous medium; an aqueous medium circuit having a circulation pump and the refrigerant-water heat exchanger, in which the aqueous medium having exchanged heat with the refrigerant in the refrigerant-water heat exchanger circulates, and the aqueous medium circuit being connected to aqueous medium devices (8, 9) configured to use the aqueous medium to perform an operation; an auxiliary heater configured to further heat the aqueous medium circulating in the aqueous medium circuit, the auxiliary heater being provided to an aqueous medium outlet side of the refrigerant-water heat exchanger in the aqueous medium circuit; a heating capability computation unit configured to compute a heating capability of the aqueous medium devices (8, 9) on the basis of an operating state quantity of constituent devices or of the refrigerant flowing through the refrigerant circuit; a circulation flow rate computation unit configured to compute a circulation flow rate of the aqueous medium in the aqueous medium circuit on the basis of the heating capability and of an outlet/inlet temperature difference, which is the difference between an inlet temperature and outlet temperature of the aqueous medium in the refrigerant-water heat exchanger; and a prediction unit configured to predict an outlet temperature of the aqueous medium in the auxiliary heater in a case where the auxiliary heater has acted, on the basis of the circulation flow rate and of heat source capability information indicative of a capacity of the auxiliary heater, wherein the compressor is a variable-capacity-type compressor, and the heat pump system further comprises: a heat source operation controller configured to cause the auxiliary heater to perform an operation in a case where the capacity of the compressor is a predetermined capacity or higher, and an outlet-side temperature difference, which is the difference between a target outlet temperature and the outlet temperature of the aqueous medium in the refrigerant-water heat exchanger, is a first predetermined temperature difference or higher. a heat source operation controller configured to cause the auxiliary heater to perform an operation in a case where the capacity of the compressor is a predetermined capacity or higher, and an outlet-side temperature difference, which is the difference between a target outlet temperature and the outlet temperature of the aqueous medium in the refrigerant-water heat exchanger, is a first predetermined temperature difference or higher.