PURPOSE: To obtain only the accurate shapes quickly by operating the deviation of an arc center and the deviation of a radius from the center of the revolving shaft of a detector and correcting the measured quantity from the next time automatically by using these.

CONSTITUTION: If the arc-shaped peripheral surface of an object to be measured is traced while a displacement detector 2 is rotated around a revolving shaft 1, the displacement signals eCWeE of the measurement point of ±P degrees are obtained from the signals of an angle sensor 4 and the detector 2. The center A of the object to be measured deviates by dy.dx from the center O of the revolving shaft and the radius thereof deviate by dr as well. The deviation values can be operated from the inclined component ey and middle high component ex of the signal curve and are in the end determined from the eCWeE. Since the distortion of the signal curve by the deviation is a sine wave with dx and a cosine wave with dy, the measurement signals from the next time are corrected with the above-described deviation values determined in the first measurement by the use of a sin potentiometer 5 and a cos potentiometer 6.