PURPOSE: To treat some stained water and protect a site against a so-called flood of water by a method wherein a flowing-out water and the removed particles are instantenously suctioned and discharged by an injection of discharge nozzle of an air motor applied in a grinding device in which a sand paper is slid under a rotation of the air motor.

CONSTITUTION: A valve lever 34 is operated, a compressed air is flowed into an air motor 4 to rotate the same. The compressed air upon completion of the work is passed through a fine hole 7a, and a nozzle small port 32 communicating with a connection port 3 of the dust suction pipe 37 up to a discharge port, then flowed into a water discharge tank 39. In turn, the water is flowed out with slightly delayed, and a long feeding passage causes a delay from a circular movement of the pat 30 and the sand paper 38, a discharge of water from the water discharge ports 23a and 23a' when a polishing is started and some dusts are produced, then the dusts are enclosed by the water to prevent them from being splashed away. The stained water is instantenously passed from the suction port 30a through the duct suction chamber 4 and the suctioned dust in-take pipe 36 and then flowed into the tank together with the discharge flow. Thus, the stained water treatment may positively be performed to prevent the site from being flooded with water.