PURPOSE: To increase a suction force largely by providing a dust suction switching device for performing opening and closing of a dust suction pipe interlocked with opening/closing action of a passage switching device so as to make it possible to eliminate dust produced in a polishing grain cleaning device by suction only when this cleaning device is operating.

CONSTITUTION: A passage switching device A is provided for reducing polishing grains directly or leading them to a polishing grain cleaning device 23 through a polishing grain cleaning passage 21. A dust suction pipe 22 connected with this polishing grain cleaning device 23 is connected with a dust suction switching device B at the other end, and opening/closing action of this dust suction pipe 22 is performed by the dust suction switching device B interlocked with opening/ closing action of the passage switching device A. That is, the dust suction pipe 22 is released only when the polishing grain cleaning device 23 is operating, so dust produced in the cleaning device 23 is eliminated by suction, and the dust suction pipe 22 is closed when the polishing grain cleaning device 23 is stopped, where only dust produced in a directing device 26 is eliminated by suction.