PURPOSE: To suppress variations in a load supply temperature and a quantity of heat by deciding the number of operating heat source units the number of increased or decreased units, varying load sharing ratios, immediately sending a start signal to the increased units, and outputting a stop signal to the decreased units at the time of a predetermined load sharing ratio.

CONSTITUTION: A heat source system 10 controls a flow rate of heat medium to be supplied from a plurality of heat source units 12-1, 12-2 to a load 14 by a plurality of primary pumps 13-1, 13-2. A heat source controller 11 for controlling the system 10 outputs a start/stop signal, a flow rate control signal from an input/output processor 15 to the system 10. A control processor 19 decides the number of operating units, outputs number-of-increased or -decreased units command of the units 12-1, 12-2, and outputs a switching command of the unit. Further, an optimum processor 20 generates a flow rate control signal for varying load sharing ratio of the increased unit and the existing operating unit or the increased unit and the residual operating unit, immediately sends a start signal to the increased unit and outputs a stop signal to the decreased unit at the time of a predetermined load sharing ratio.