PURPOSE: To prevent center displacement due to the self-weight of a grinding wheel by reducing the self-weight of the grinding wheel by friction force and mount the grinding wheel on a spindle shaft in a condition in which accurate centering is performed.

CONSTITUTION: A piston 22 energized by a spring 23 is provided on a grinding wheel 2. When the grinding wheel 2 is attached on a spindle shaft 1, a bolt 3 is tightened up to a halfway to compress the spring 23 so as to produce friction force between a pad 22a and a surface 12a of a flange section. In this condition, the grinding wheel 2 is turned by 180 degrees to increase pressure of a hydraulic chamber 13 while all the self-weight of the grinding wheel 2 does not act on a shaft section 11. The centering of the grinding wheel 2 for the spindle shaft 1 is performed, and the bolt 3 is tightened completely to fix the grinding wheel 2 on the spindle shaft 1.