PURPOSE: To improve the delivery quantity measured value by calculating the delivery quantity with the preset arithmetic equation specified based on the predetermined characteristics of a delivering machine according to the driving quantity of the delivering machine and the pressure difference before and after the delivery and using the value as the delivery quantity of the delivering machine.

CONSTITUTION: The pressure difference (static pressure) ΔP between the pressure on the suction side A of a blower 1 controlled by a speed changing device 2 for its rotating speed and the pressure P2 on the discharge side B is detected by a pressure difference detector 5. The rotating speed N of the blower 1 specified by the speed changing device 2 and the static pressure ΔP from the detector 5 are inputted to a wind quantity arithmetic unit 6, and the wind quantity Q is calculated by a preset equation. The high-precision wind quantity measured value not affected by the wind speed is obtained at a low cost.