PURPOSE: To detect wear of grooves and evaluate its status quantitatively by calculating the position of grooves and converting the calculated position and original image into a unit and waveform data respectively so as to calculate the degree of wear.

CONSTITUTION: A piston to be evaluated is mounted on a rotating stage, for example, and the developing picture of outer circumferential surface of the piston is sensed by an optical system and CCD sensor while the piston is rotated, so as to prepare original image data. Then, the position of grooves which is prepared at the time of manufacturing the piston is calculated based on the original image data. That is, the position is estimated from the difference of density in the original image data. Further, the data is converted into a waveform data along an axis crossing the worked groove at a right angle. The waveform data is processed at a unit of width of respective grooves that is calculated from the position of the grooves. In addition, the contact strength for every groove width is calculated based on the waveform data. Since the picture data has various gradations, the contact strength can be calculated based on the difference of its light and darkness.