A device for detecting any defects in single faced corrugated fiberboard while it is running. A light is irradiated to the corrugated surface of the fiberboard and the striped image is scanned by an image pick-up unit and the picture signal is converted to binary-coded signal to discriminate the number of stripes or corrugations. The number is counted by a counter and compared with a normal number by a microcoputer. If there is too large a difference between them, the scanned part is judged as defective. This device can detect any defects in a running web at a sufficiently high speed to follow the line speed of a corrugating machine.