PURPOSE: To control a discharge pressure of a blower to a set pressure and to reduce power consumption by arranging a pressure detecting sensor in an air supply passage, obtaining a deviation of the detected pressure from the set pressure, and inputting the deviation to an inverter to control number of revolutions of the blower.

CONSTITUTION: A pressure detecting sensor 5 is arranged in an air supply passage 2 to a burning furnace of a blower 1. An air supply pressure is detected by the sensor 5, and this detection signal is input to a deviation output unit 7. On the other hand, the unit 7 obtains a deviation of a pressure input from the sensor 5 from a pressure set by a pressure setter 8, and outputs the deviation to an inverter 9. The inverter 9 so controls to feed back the number of revolutions of the blower 1 based on the deviation as to become a set discharge pressure. Thus, the number of revolutions of the blower 1 is reduced at the time of a low flow rate to reduce power consumption, a flow rate control is simplified, and an air supplying pressure is stabilized.