PURPOSE: To correct rotation accuracy, particularly an angular error, and to attain high rotation accuracy with a simple structure by screwing conical adjusting screws into conical adjusting screw holes formed on the outer periphery of a rotary holder clamping and holding a workpiece or a tool.

CONSTITUTION: Four conical adjusting screw holes 8-11 are formed at intervals of about 90° in the peripheral direction on the outer periphery of a holder 1 corresponding to an adjusting ring 3, and conical adjusting screws 12-15 are screwed into them. The holding angle of a workpiece or a tool 7 can be changed via the distortion of the holder 1 by properly setting the intervals, properly setting the tapered degrees of the adjusting screw holes 8-11 and adjusting screws 12-15, and screwing the adjusting screws 12-15. The angular error of the rotation accuracy of the workpiece or tool 7 can be adjusted or corrected by measuring the holding angle at the measurement point P of the workpiece or tool 7 portion on the outside of the holder 1, screwing the selected adjusting screws 8-11, and adjusting the quantities.