The purpose of the present invention is to maintain a water supply temperature close to a target water supply temperature at the time of a change in the number of heat source machines operating. A heat source system of the present invention anticipates a case where a prescribed minimum flow rate is set for heat source machines that are subject to addition or removal, and calculates, as compensation temperatures, hot and cold water outlet temperatures of the heat source machines so that the water supply temperature in such a case will match a target water supply temperature, and changes the hot and cold water outlet temperature settings of the operating heat source machines to the compensation temperatures. After that, the heat source machines that are subject to addition or removal are either started up or stopped, and the set flow rate for the heat source machines that are subject to addition or removal is set to the minimum flow rate.