PURPOSE: To automatically detect a break of a corner of a constructional material through image processing, particularly, to enhance the detecting accuracy to a break of a small area ratio.

CONSTITUTION: A coordinate Z of a virtual crossing point is obtained from a line A connecting a plurality of boundary coordinates (a), (b) in the X-axis direction on an image MV by a photographing device and a line B connecting a plurality of boundary coordinates (c), (d) in the Y-axis direction. A detecting segment D closest to the coordinate Z and connecting a plurality of edge coordinates appearing on the image MV is obtained by plotting a plurality of points in the X-axis or Y-axis direction. The detecting segment D is compared with a comparative segment Ae (Be) which is a natural boundary of the corner on the line in the X-axis or Y-axis direction. If the segments are agreed with each other, no break is present in the material. If the segments are not agreed, it is so detected that the corner is broken.