To prevent the deterioration of components for ignition such as the abrasion of a spark rod.

The control program, which enables backup control to temporarily raise the combustion position of any boiler among boilers under combustion when starting the combustion of any boiler in a boiler group, makes backup first control executable to, when it is determined that it is necessary to start the combustion of any boiler, and the combustion position of the boiler in which backup control is executable is further raised for increase, transmit a pre-purge signal to the boiler whose combustion is started and backup second control executable to, when such transfer conditions that the detection pressure of a pressure detector does not rise beyond a set value are satisfied even after the lapse of a set time since the backup first control is started, transmit an ignition signal to the boiler under pre-purge to transfer to low combustion, and to, when the transfer conditions are not satisfied, and it is determined that it is not necessary to start the combustion of any boiler, stop the pre-purge.