The process and the device for measuring the displacement of a web within a multi-color rotary printing press by means of a mark simultaneously printed with each color in an area free of any printing characterized by the mark passing beneath the photodetector arrangements for generating a signal. The method and the device are further characterized by the fact that the pair of photodetectors are arranged side-by-side along a crosswise line with the edges of the field for each detector arrangement being positioned above the web at the expected position for the passage of the center of the crosswise symmetrical mark and the crosswise displacement of the web is determined by the difference between the intensity variations of the signals created by each of the two photodetectors when the mark passes therethrough.