PURPOSE: To rapidly and easily change a grinding wheel by mounting a roughing grinding wheel consisting of a cylindrical body on a flange plate, and mounting a finishing grinding wheel consisting of a cup body on a rotary shaft respectively.

CONSTITUTION: When a bolt 28 is drawn out of a flange plate 27 by a wrench, a cylindrical body 29 is detached from this flange plate 27, and a roughing grinding wheel G1 is removed from the mounting position. Because the inner diameter part of the cylindrical body 29 is larger than the outer diameter part of a finishing grinding wheel G2, the finishing grinding wheel G2 need not be removed to allow smooth attaching/detaching operation. When the finishing grinding wheel G2 is changed, a stop nut 34 is detached from a rotary shaft 11, and a cup body 32 is drawn out in the forward direction. The roughing grinding wheel G1 need not be removed due to the relationship between the outer diameter part of the finishing grinding wheel G2 and the inner diameter part of the roughing grinding wheel G1, allowing the smooth attaching/detaching operation.