PURPOSE: To simply work a concentric portion and an eccentric portion by rotatably inserting a hydraulic mandrel having an eccentric hole in an eccentric hole of an auxiliary main spindle, and rotating the hydraulic mandrel to be automatically positioned in a zero-eccentricity position and in an eccentric position.

CONSTITUTION: The phase of an adjusting ring 15 to an auxiliary main spindle 7 is adjusted to set the amount of eccentricity, and a hydraulic mandrel 8 is rotated in an eccentric hole 70 of the auxiliary main spindle 7, and positioned to a preset amount of eccentricity, with a positioning lever 17a pressed to a stopper B. Pressure oil is supplied in a rear chamber of a hydraulic cylinder 18 to clamp the hydraulic mandrel 8 in an eccentric hole 7a of the auxiliary main spindle 7, and simultaneously an eccentric hole 8a is closed to clamp a workpiece W and work an eccentric portion. On completion of working the eccentric portion, the hydraulic mandrel 8 is reversed and positioned in a zero- eccentricity position, with the positioning lever 17a pressed to a stopper A. Thus, the hydraulic mandrel 8 is put in a clamp state to work a concentric portion.