PURPOSE: To measure the absolute shape precision of a workpiece by revolving a thickness measuring mechanism for the workpiece which uses the laser beam cutting method around an axis parallel to a light beam surface and allowing the light beam to travel within the plane which crosses nearly right angle for the rotary shaft of a rotary spindle shaft.

CONSTITUTION: The thickness of a workpiece 3 is measured by using a thickness measuring mechanism using the laser beam cutting method, and the above- described measuring mechanism is installed on the carriage of a machine tool, and the coordinate value of each carriage and the diameter of the workpiece are measured continuously. At this time, in order to avoid the installation error of the thickness measuring mechanism, rotary mechanisms 13a, 13b, and 21 are installed in the lower part of the thickness measuring mechanism 1, and the thickness measuring mechanism is adjusted so that the measured thickness becomes min. Further, when the distance between the position of the cutting edge of a cutter and the thickness measuring mechanism is measured, groove working is carried out for a part of the workpiece, and the coordinates of the step difference is measured by the thickness measuring mechanism.