PURPOSE: To classify a particulate material under low pressure with good accuracy by enabling classification only by air stream action, in a classifier having an inflow pipe provided to the outside thereof and provided with a rotary stream generation part therein, by providing a particle cutting cylinder while mounting a slide damper to a part of the inflow pipe.

CONSTITUTION: An inflow pipe 2 is attached to the outside of a classifier from the tangential direction thereof and a slide damper 3 is provided to a part of the inflow pipe 2 so as to be obliquely retracted to the outside of said pipe 2. On the other hand, a rotary stream generation part 11, a ring shaped dust collection part and a dust collection hopper 12 are successively provided in front of a base cylinder 1 having a high pressure air jet pipe 6 received therein. A conical receiving cylinder 13 is connected to the rear part of the aforementioned rotary stream generation part 11 and a particle cutting cylinder 16 is provided to the intermediate part of the receiving cylinder 13 while a pressure release pipe 17 is provided between the hopper 12 and the central part of the base cylinder 1.