Feedwater to be supplied to a feedwater tank via a feedwater path is passed through a waste heat recovery heat exchanger, a supercooler, and a condenser in sequence. A heat source fluid such as heat source water is passed through an evaporator and the waste heat recovery heat exchanger in sequence. The waste heat recovery heat exchanger is an indirect heat exchanger between the feedwater supplied to the feedwater tank via the feedwater path and the heat source fluid having passed through the evaporator. The supercooler is an indirect heat exchanger between the feedwater supplied to the feedwater tank via the feedwater path and a refrigerant supplied from the condenser to an expansion valve.