According to a measuring method, shapes of two mechanical parts of a measuring object are shot by a camera at their respective axial both ends so that image information of the shapes are obtained. Then a center coordinate of the detected shape is respectively calculated from the image information, and positions of the axial lines for the two mechanical parts are calculated from the calculated center coordinates, and finally the calculated position of the axial line is compared with the calculated position of the other axial line in a certain determination area, to determine the coaxial relation between the two mechanical parts.