To increase a heat recovery amount in an evaporator while preventing breakage of a compressor by liquid compression.SOLUTION: A feedwater heating system 1 includes a vapor compression-type heat pump circuit 10 in which a compressor 11, a condenser 12, an expansion valve 13 and an evaporator 14 are annularly connected by a refrigerant circulation line L9, and in which hot heat is taken out by the condenser 12 by driving the compressor 11; refrigerant flow rate adjustment means for adjusting a flow rate of a refrigerant R flowing in the heat pump circuit 10; a heat source temperature sensor detecting a temperature of a heat source fluid exchanging heat with the refrigerant R in the evaporator 14: and control means for controlling the refrigerant flow rate adjustment means. The control means determines a target superheating degree according to a detection temperature of the heat source temperature sensor, and controls the refrigerant flow rate adjustment means so that a superheating degree of the refrigerant R flowing into the compressor 11 becomes the target superheating degree.SELECTED DRAWING: Figure 1