An object is to provide a heat source system capable of realizing energy saving. In a heat source system including: a chilled-water main control heat pump (1) having a lower-stage evaporator (9) that cools chilled water and a lower-stage condenser (7) that dissipates heat into cooling water, and outputting the chilled water cooled in the lower-stage evaporator (9) to an external load; and a hot-water main control heat pump (51) having a higher-stage evaporator (59) to which heat is provided by medium-temperature water that has been heated with the exhaust heat from the lower-stage condenser (7) and a higher-stage condenser (57) that heats high-temperature water, and outputting the high-temperature water heated in the higher-stage condenser (57) to an external load; a lower temperature limit of the cooling water is set on the basis of a set temperature of the high-temperature water.