To prevent output of a command for changing a combustion amount to a boiler which is not necessary to change the combustion amount and output an reverse command immediately after that.

A multiple boiler installation system is equipped with a number of boiler controller 3 that determines a combustion state each of a plurality of boilers 1 installed therein according to the vapor pressure at a vapor gathering section 4 and controls the number of the boilers for the necessary number of boilers in combustion. When changing output of a combustion state is issued based on a vapor pressure variation, a combustion command for change of combustion state is not output until a predetermined time T passes after the vapor pressure change is detected. If the change direction of the vapor pressure is reversed within the predetermined time T, the combustion command of combustion state change based on the vapor pressure variation detected before the change direction of the vapor pressure is discarded.