PURPOSE: To permit the garden stone of a desired size to be readily prepared by grinding a rock in a rotating drum before washing and the like.

CONSTITUTION: The mined rock S is cut into a desired size, and thrown into the rotating drum 10, into which a proper quantity of gravel and sand are additionally thrown. Then a motor 3 is started to rotate the rotating drum 10 at a low speed, and at the same time, water is supplied from a supply port. The water supply is stopped when a given quantity of water has been supplied. The rotating drum 10 is still continuously rotated for a given period of time to grind the rock S by the contact with the gravel, sand and water so that the surface of the rock S is allowed to be polished. After that, the ground rock S is discharged and let to fall from an inlet/ outlet port 19 together with the ballast, sand and water, and then separated from them, washed and finished as the garden stone.