In a classifier including a rotary disc provided on a lower end of a vertical rotary shaft depending from the top of a hopper-type casing, a horizontal dispersing disc is provided directly below the top of the casing, a circular collision plate is spaced from the outer circumference of the horizontal dispersing disc, and a plurality of vortex adjusting members are secured at their upper ends to the horizontal dispersing disc and at their lower ends to the rotary disc, and a leakage prevention apparatus is provided which comprises a plurality of dispersing and classifying vanes spaced from the outside of the vortex adjusting members and the inner sides of the collision plate. The dispersing and classifying vanes depend from the under surface of the horizontal dispersing disc for an appropriate length. Furthermore, the leakage prevention apparatus can also comprise a ring for preventing the introduction of coarse powder which is provided on the outer circumference of the horizontal dispersing disc, the ring depending from the under surface of such horizontal dispersing disc for an appropriate length.