PURPOSE: To permit easy centering by operating the deviation of an arc center and the deviation of a radius from the center of the revolving shaft of a detector from the detected values of radial displacements at three measurement points.

CONSTITUTION: If the arc-shaped inside peripheral surface of an object to be measured is traced with a measuring element 6 while the displacement detector 5 of a shape measuring device is rotated around a revolving shaft 1, the displacement signals eCWeE of the measurment points CWE of ±P degrees are obtained from the signals of an angle sensor 7 and the displacement detector. The geometrical center A of the object to be measured deviates by dx.dy in the X-Y directions from the center O of the revolving shaft and the radius thereof deviates 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 dx=K{eD-(eE+eC)/2}, dy=J(eE-eC), dr=eD-dX.