PURPOSE: To-held a shaft member of various different sizes firmly so as to drive rotatable by providing a displacing mechanism to move rotation transmitting mechanisms and a center holding mechanism integrally in the axial direction of the shaft member.

CONSTITUTION: In the condition holding rotatable both ends of a shaft member 12 through a center holding mechanism 14, the outer periphery of the shaft member 12 is grasped with plural grasping member through the cylinders 74a to 74c of rotation transmitting mechanisms 18 and 20. Then, a rotary hollow main shaft 72 and a shaft member 12 are driven to rotate integrally, and a grinding process is applied to a specific position of the outer periphery of the shaft member 12 through a grinding wheel 16. In this case, when the long size shaft member 12 is grinding processed, the rotation transmitting mechanisms 18 and 20, and the center holding mechanism 14 are displaced in the axial direction of the long size of shaft member 12 by a displacing mechanism 26, and the shaft member 12 is held firmly corresponding to the grinding position.