The mounting section 27 is made of a material, such as a brush and sponge rubber, soft enough to be able to keep a plane posture of the metal sheet 30 as shown in FIG. 2A. [0044] Next, the metal sheet 30 placed on the mounting section 27 is photographed with a camera (not shown) and the image is subjected to image processing in an image processor (not shown) to acquire the amount of positional dev