PURPOSE: To perform stable classification with high accuracy by suppressing the rise of the concn. of the particles within a classifying area.

CONSTITUTION: The separation core 22 having a truncated cone shape provided to the lower part of a classifying chamber 20 is separated into the guide part 22a of the axial core part thereof and the guide part 22b of the outer peripheral edge part thereof so as to provide a definite interval 22c between both guide parts. The radius of the axial core part 22a is made larger than that of the rotary circle generated when particles of a classifying limit are rotated by the free vortex in the classifying chamber 20.