PURPOSE: To decrease a cleaning work caused by dirt of a polishing material by separating the polishing material by a rotary drum and grinding the polishing material in the drum in which a grinding stone is enclosed, taking out generated dust to the outside of the drum and cleaning the surface of the polishing material.

CONSTITUTION: In a round U-pipe 2 installed in a state inclined a little, a waveform groove 4 and a hole 5 are formed, and in its inside, a spiral 17 is supported by a spiral supporting shaft 9 attached to its center part, at a moderate clearance. To the outside peripheral part of a semicircular U-pipe 2, a rotary drum 3 with minute holes is fixed, and rotated by a rotary plate 13 in the outside peripheral part of the spiral supporting shaft 9. Between the semicircular U-pipe and the rotary drum 3 with minute holes, a grinding stone is enclosed, and in the outside periphery of a rear part of an outlet part, a polishing material discharge port 8 is formed. To the outlet part of the semicircular U-pipe 2, an inverted L-glange part 7 whose diameter is smaller than the outside peripheral part is attached, and PACHINKO balls pass through this part since its diameter is large and flow out to a ball discharge chute 10, and is obstructed since a polishing material is minute and is discharged from the hole 5.