PURPOSE: To remove the side face deflection of a grindstone by forming an annular elastic deflection part between a slit in the circumferential direction and a grindstone abutting face, and fitting a bolt freely movable in a spindle axial direction to a flange main body in order to press this deflection part to the grindstone abutting face side while facing it to each slit.

CONSTITUTION: In the case that the forming accuracy of a grindstone 1 is bad, the grindstone 1 is fitted by being inclined to the shaft center of a spindle shaft 2 and a side face deflection is generated according to the rotation of the spindle shaft 2, the upper bolt 6 and lower bolt 6' screwed with the flange main body of each of flange assembly bodies 4, 4' are advanced and the lower bolt 6 and upper bolt 6' are retreated. Thus the annular elastic deflection parts 7, 7' of each flange assembly body 4, 4' are deflected to the grindstone 1 side, the grindstone 1 is revolved in the left round and the side face deflection of the grindstone 1 is removed.