PURPOSE: To effectively correct an angular error and to improve rotational accuracy by forming conical adjusting screws on the outer peripheral section of a rotary holder clamping and holding the held section of a work or a tool, and screwing the conical adjusting screws.

CONSTITUTION: Conical adjusting screw holes 8-11 are formed at four positions on the outer peripheral section of a rotary holder 1 at the corresponding portion of an adjusting ring 3 clamping and holding the held section of a work or a tool, and conical adjusting screws 12-15 are screwed into the adjusting screw holes 8-11. When the conical adjusting screws 12 or 13-15 are strongly screwed into the adjusting screw holes 8 or 9-11, a kind of strain occurs on the rotary holder 1, the strain is changed according to the screwing quantities and screwing positions of the adjusting screws 12-15, thus the rotational attitude of the work or the tool is changed, thereby the rotational accuracy, particularly the angular error, of the work or the tool is effectively corrected, and high rotational accuracy can be obtained.