PURPOSE: To stop an troublesome operation so as to economize personnel expenses by providing such constitution that a blower is operated automatically at a high speed compulsorily for a certain preset period at starting time and/or every lapse of a prescribed time after starting so as to discharge foreign matters inside a duct.

CONSTITUTION: Target pressure of a pressure setting device 10 and real pressure detected by a pressure detector 12 through a pressure sensor 30 are inputted to a comparator 16, and a comparison result thereof is calculated by a PI calculator 18 so as to input a speed command to an invertor 20. Thereby, a static pressure constant control operation is carried out so that pressure of an electrical motor 24 for a blower 26 becomes the target pressure. And at starting time and/or every lapse of a prescribed time after starting, the invertor 20 is controlled so as to operate the electric motor 24 at a high speed for a certain preset period, independent of a speed command from the PI calculator 18 so that the cleaning operation for discharging foreign matters is carried out.