To enable even an inexperienced worker to easily, quickly and accurately determine pass or fail on width of a mating part as well as on existence of a piston ring and circumference direction position of the mating part, in relation to a piston before being assembled in an engine for an automobile or the like.

A piston 5 is rotated around a axis thereof under a condition where laser beam LB in a direction inclined in relation to a piston radial direction is emitted from a side position of the piston 5 toward an assembly position of the piston ring 21, an image of a round of outer circumference thereof is captured by a CCD camera 16 from the piston side position. Consequently, existence of the piston ring 21, circumference direction position of the mating part of each piston ring are confirmed and pass-fail of width of mating part of each piston ring is determined by data processing of captured image.