To swiftly start, at the time of power recovery after a power failure, heat source machines, the number of which is equal to the number of machines before the power failure, without including an uninterruptible power supply in an apparatus for controlling the number of machines that is adapted to control the number of heat source machines. There is provided a heat source system, in which a host control device (20) includes a nonvolatile first storage unit (22) that stores the number of heat source machines in operation immediately before the power failure. When power is recovered, control on the number of heat source machines at the time of power recovery is performed in accordance with the number of heat source machines stored in the first storage unit (22).