PURPOSE: To detect a winding dislocation size at the electromotive section of a spiral structure precisely at all times, regardless of material or color, by cross-photographing the surface of an inspection object by use of a pair of CCD cameras.

CONSTITUTION: The prescribed zones of the stacked spiral structure edge of a positive electrode layer 1, an insulation layer 3a, a negative electrode layer 2 and another insulation layer 3b, are first cross-photographed with a pair of CCD cameras 4a and 4b, and the images of the edge are thereby obtained. In this case, the perspective image of the layer 3a through the camera 4a changes, depending on the projection length of the inner layer 3b. Also, a positive electrode real field size (d) and perspective image size d' corresponding to the photographed zones of the layer 1 also changes, depending on the length. The adjacent layers 3a and 3b, however, look as if integrated, and the size (d) of the layer 1 cannot be obtained. On the other hand, the perspective image of the layer 3a through the camera 4b is not influenced by the layer 3b and the size (d) of the layer 1 can be obtained. In addition, regarding the sizes (d) of the layer 1 through the cameras 4a and 4b, sizes (d) or d' among input images from the cameras 4a and 4b are compared with each other, and a smaller size is adopted.