The objective is to achieve stable output from a heat source system that includes heat source machines which utilize unused heat from sewage water or the like as the heat collection source, while using the unused heat as much as possible as the heat collection source. This heat source system is equipped with: first heat source machines, which utilize unused heat from sewage water or the like as the heat collection source; second heat source machines, which utilize a heat collection source other than unused heat; and a control device (10). The control device (10) calculates the maximum amount of heat that can be output by the first heat source machines, and the maximum load rate that can be output by the first heat source machines, and in accordance with the result of those calculations, the control device changes the increased/decreased-quantity priority and the increased/decreased-quantity load rate threshold value for the heat source machines, and controls the increasing/decreasing of the quantity of the heat source machines.