A heat pump system which is provided with a heat pump unit having a compressor and which is arranged for supplying a fluid heated by the heat pump unit to a fluid-using device that uses the fluid, the heat pump system comprising: an inflow temperature measurement part for measuring a temperature of the fluid flowing into the fluid-using device, as an inflow temperature: an outflow temperature measurement part for measuring a temperature of the fluid flowing out from the fluid-using device, as an outflow temperature; characterized in that the heat pump system comprises a change rate calculation part for calculating a change rate per predetermined time of the inflow temperature measured by the inflow temperature measurement part, as an inflow temperature change rate sequentially since start of operation of the compressor, and the change rate calculating part is arranged for calculating a change rate per predetermined time of the outflow temperature measured by the outflow temperature measurement part, as an outflow temperature change rate sequentially since start of operation of the compressor; a minimum temperature setting part for setting, as a minimum temperature, the inflow temperature with which a difference between the inflow temperature change rate and outflow temperature change rate calculated by the change rate calculation part becomes not more than a preset change rate threshold; a target temperature setting part for setting a temperature not less than the minimum temperature set by the minimum temperature setting part, as a target temperature; and a control part for controlling the compressor of the heat pump unit such that the temperature of the fluid flowing into the fluid-using device becomes equal to the target temperature set by the target temperature setting part. an inflow temperature measurement part for measuring a temperature of the fluid flowing into the fluid-using device, as an inflow temperature: an outflow temperature measurement part for measuring a temperature of the fluid flowing out from the fluid-using device, as an outflow temperature; characterized in that the heat pump system comprises a change rate calculation part for calculating a change rate per predetermined time of the inflow temperature measured by the inflow temperature measurement part, as an inflow temperature change rate sequentially since start of operation of the compressor, and the change rate calculating part is arranged for calculating a change rate per predetermined time of the outflow temperature measured by the outflow temperature measurement part, as an outflow temperature change rate sequentially since start of operation of the compressor; a minimum temperature setting part for setting, as a minimum temperature, the inflow temperature with which a difference between the inflow temperature change rate and outflow temperature change rate calculated by the change rate calculation part becomes not more than a preset change rate threshold; a target temperature setting part for setting a temperature not less than the minimum temperature set by the minimum temperature setting part, as a target temperature; and a control part for controlling the compressor of the heat pump unit such that the temperature of the fluid flowing into the fluid-using device becomes equal to the target temperature set by the target temperature setting part. an outflow temperature measurement part for measuring a temperature of the fluid flowing out from the fluid-using device, as an outflow temperature; characterized in that the heat pump system comprises a change rate calculation part for calculating a change rate per predetermined time of the inflow temperature measured by the inflow temperature measurement part, as an inflow temperature change rate sequentially since start of operation of the compressor, and the change rate calculating part is arranged for calculating a change rate per predetermined time of the outflow temperature measured by the outflow temperature measurement part, as an outflow temperature change rate sequentially since start of operation of the compressor; a minimum temperature setting part for setting, as a minimum temperature, the inflow temperature with which a difference between the inflow temperature change rate and outflow temperature change rate calculated by the change rate calculation part becomes not more than a preset change rate threshold; a target temperature setting part for setting a temperature not less than the minimum temperature set by the minimum temperature setting part, as a target temperature; and a control part for controlling the compressor of the heat pump unit such that the temperature of the fluid flowing into the fluid-using device becomes equal to the target temperature set by the target temperature setting part.