To suppress the occurrence of erroneous detection even when the flaws of a plurality of objects to be detected with different sizes are continuously detected by comparing the luminance of a pixel in a comparison area provided in a central part with the luminance of a pixel in a comparison area provided in an edge part and setting the object area of flaw detection in the image of the object to be detected on the basis of obtained comparison results.

A camera 14 of a 1st image pickup device 11 picks up the upper part of an object S to be detected directly and a lateral image indirectly and stores the obtained image signal in a 1st frame memory 4. An image extracting part 7 binarizes the obtained image with a preliminarily set threshold and specifies the contour of the object S included respectively in the upper image and both lateral images of the object S on the basis of an obtained binarized image. Then, the image of the object S is respectively extracted from the upper image and the both lateral images before binarization in the memory 4 on the basis of the specified image. A detecting part 8 detects the existence/ absence of a flaw and stores the results in a memory 10.