PURPOSE: To obviate long-term variations in sliver weight by correcting open- loop or closed-loop control in accordance with the ambient air humidity.

CONSTITUTION: An measuring device 30 is installed in a spinning hall, measuring the absolute air humidity, and an electronic clock pulse generator 32 is connected to the storage device 33. Relating to a division calculator 37, a reference value from a reference value setting unit 39 is divided by the average value from an average value forming unit 35, and the result is sent to a multiplier 41. The multiplier 41 multiplies an adjustment value from an adjusting unit 27 by a signal value from the division calculator 37, varying a fan driving motor 28. A measurement originating unit 26 mounted on a drawer roller 14 transmits an electrical signal in proportion to a deviation from a target position. This signal reaches the adjusting unit 27. A pressure P1 changes by the change in the number of revolution of the motor 28.