The purpose of the present invention is to avoid frequent repetition of an increase/decrease stage caused by the inclusion, in a heat source system, of a machine the capacity of which has been degraded. A heat source system of the present invention is provided with a plurality of heat source machines. A higher-level control device (20) controls each of the heat source machines so that a heat medium delivery temperature, which is the temperature of a heat medium being supplied to an external load, will be a set temperature. The higher-level control device (20) is provided with a number-of-machines control unit (22), a degraded machine detection unit (24), and a priority ranking modification unit (25). The number-of-machines control unit (22) controls the number of heat source machines in accordance with operational priority ranking information in which each of the heat source machines and an operational priority ranking are associated together. The degraded machine detection unit (24) detects, as a capacity-degraded machine, a heat source machine that satisfies a predetermined capacity degradation condition, from among operating heat source machines. The priority ranking modification unit (25), if a capacity-degraded machine has been detected, modifies to a lowest position the operational priority ranking of the capacity-degraded machine in the operational priority ranking information.