PURPOSE: To enable labor-saving, efficient cutting of stone by conducting dressing when the cutting performance of a rotary blade is lowered during automatic cutting and controlling the dressing to cut the stone with relatively small depth of cut and low feed speed.

CONSTITUTION: During automatic cutting of stone W, the current of a main shaft motor 7 is continuously detected so that the depth of cut and feed speed of a rotary blade 8 can be controlled corresponding to the current. When the current of the main shaft motor 7 exceeds a given value, the dressing of the rotary blade 8 is started automatically. When the current is lowered below the given value, the dressing is stopped automatically so that the rotary blade 8 is controlled automatically to conduct ordinary cutting operation. The dressing is carried out to cut the stone W with relatively small depth of cut and feed speed.