PURPOSE: To carry out an online detect of a scratch in the cold rolling process by eliminating water drop which maker an extremely large disturbance in signal processing. CONSTITUTION: The image of a plate being rolled is picked up by a camera so that the image partially overlaps in the previous and later images to obtain a plurality of image signals, positions Qn and Qn-1 of an abnormally bright or dark point on the substrate are detected based on the image signal, the position of the abnormally bright or dark point is compensated by the traveling distance Dn and Dn-1 of the plate between the image pick-up intervals and is compared with the position of the abnormally bright or dark point of adjacent images, and then it is judged that the point is a scratch and water drop when it is located at the same position and is not at the same position, respectively.