The object of the present invention is to provide a compact lightweight heat pump hot-water supply system. The hot-water supply system heats water through heat exchange between high-temperature refrigerant of a heat pump circuit and water of a water circulating circuit. The system is an instantaneous water heater unit for instantaneously heating, at a water heat exchanger, water supplied through a water supply pipe and supplying hot water to a using end unit. In this system, therefore, large-capacity hot-water storage tank in conventional systems is no longer needed. Sufficient heat of condensation to heat water can not be produced, because pressure conditions of the heat pump circuit is not stabilized just after the start of operation. Therefore, during a short period of rising time, hot water stored in a hot-water supply tank and water from the water heat exchanger are mixed and the mixture is supplied to keep the hot water at a preset temperature. It is, therefore, possible to downsize the hot-water supply tank.