Abstract: Images of successive portions of a surface of a strip are formed and converted as analog signals. Signals typical of each image are stored in an image storage whereas the analog signals relating to each image are converted in numerical form by splitting up the image into image elements. Each of the image elements is given a numerical value, and then filtered, in order to sense the relative variations of the numerical values of the image elements. To determine whether or not to deliver a defect-sensing signal in terms of the sensed variations, the filtered signal is scanned. The typical image is stored in response to the transmission of the defect signal. Every read-in information is stored and displayed at a check station long enough for a visual check of the image it represents.