PURPOSE: To maintain work machining precision at a high value and to compact a drive mechanism for an outer wheel rotary shaft by providing the outer wheel rotary shaft supported on the outer peripheral surface of the stationary shaft, having a central hole, through a bearing, a drive shaft inserted and disposed in the central hole, and a disc member for intercoupling the one end of the drive shaft and the outer wheel rotary shaft.

CONSTITUTION: A central hole 12 is formed in the center of a stationary shaft 10, and a drive shaft 14 to transmit the rotational drive force of a motor 16 is inserted in the central hole 12. A disc member 20 is fixed to the tip of the drive shaft 14, a tooth part 48 formed in the outer periphery of the disc member 20 is contained in a recessed part 42 formed in the tip part of a main spindle 18 and fastened by means of a screw member 44. Since a work face plate 24 can be attached directly to a tip surface 40 of the main spindle 18, a distance size between a bearing position and the mounting position of the work, i.e., an overhang amount, is minimized. An error in mounting orientation and the mounting position of the work is reduced by an amount equivalent to minimization of the distance size therebetween, and in turn machining precision is improved.