PURPOSE: To eliminate a wasteful consumption of the power of the actuator to drive a blower and ensure the stability of a combustion condition of the burner at the time of a low load combustion.

CONSTITUTION: The flow rate of air which is fed to the furnace of a boiler main body 1 is controlled in such a manner that when the boiler load is higher than a specified load, the revolution number of a blower 3 is adjusted under a condition in which the opening of a damper 8 is held at an approximate uniform opening being larger than a specified opening. When the boiler load is lower than the specified load, the quantity of air to be fed to the furnace of a boiler main body 1 is controlled by adjusting the opening of the damper 8 under a condition in which the blower 3 is held at an approximate uniform revolution number by which the combustion of the burner does not become unstable even when there is no damper.