To prevent fluctuation of a gas pressure or an air distribution amount in a furnace when a rotational frequency command of an induced draft fan according to boiler load in an invertor operation and an actual rotational frequency command input to an invertor power source are different from each other.

The rotational frequency command 33 of the induced draft fan according to the boiler load and the actual rotational frequency command 18 input to the invertor control device 10 are compared 91, 92, 94, and a damper opening correction command 52 is multiplied by a correction coefficient 81 according to a result of the comparison a/b to correct a damper opening command 17.