PURPOSE: To adjust a classifying point at the time of the operation of an air classifier by adjusting the angle of the guide vane provided to the classifier.

CONSTITUTION: Many guide vanes 10 are provided between the upper ring provided to the lower end of an upper casing and the lower ring 9 provided to the upper end of a lower casing so as to provide a required interval in the peripheral direction of the casings and the central parts of the guide vanes 10 are supported in a freely rotatable manner by the support pins 16 supported at both end parts thereof by the upper ring and the lower ring 9. Adjusting rings 18 are attached to the annular recessed parts provided to the outer peripheral part of the rear surface of the upper ring in a freely rotatable manner and guide holes 25 inclined in the peripheral direction are provided to the rear surfaces of the adjusting rings 18. The guide pins 26 inserted in the guide holes 25 are fixed to the outer end parts of the guide vanes 10 and the guide vanes 10 are simultaneously rotated around the support pins 16 by the rotation of the adjusting rings 18.