PURPOSE: To conduct wet grinding effectively by supplying cooling water to a grind stone through a rotor through-hole and the hollow setion of a shaft in the air grinder for wet grinding.

CONSTITUTION: Compressed air is supplied into a cylinder 2, a rotor 4 mounted into the cylinder 2 is turned, and the grind stone 10 held to the rotor 4 by means of a chuck 9 is rotated, thus conducting grinding working. It this case, cooling water fed from a feed valve 28 passes through the rotor through-hole 4c and the inside of the hollow shaft 9a of the chuck 9 and is discharged from the nose section of the chuck 9, and is supplied onto the grind stone 10.