PURPOSE: To find a true outside diameter by joining a plurality of measuring errors within an outside measuring range of a detector, finding a calibration curve of the detector and applying a correction value obtained from the calibration curve to an outside diameter measured value of a measurement object of an arbitrary outside diameter.

CONSTITUTION: Movement of a frame 4 having fixed an outside diameter measuring device 3 is controlled with a controller 8 and a signal transmitted from the device 3 is data-processed 7. The device 3 is moved up to a position of a calibration gage 6, outside diameter measurement is performed for gages 6 with different outside diameter values respectively and a differences between an outside diameter measured value and the outside diameter value of the gage 6 is plotted for each outside diameter value. Each plotted point is joined with a least square method and a calibration curve can be obtained. A series of data processing such as plotting of measured data and calculation of the calibration curve can be automatically, performed with a computer. Next, a correction value is found from the calibration curve for the outside diameter measured value, applied thereto and a true outside diameter value can be obtained.