Method and device for monitoring a moving web, such as a wire in a paper machine, a felt, or a material web, such as a board or paper web. A bar of radiation beams is directed at the face of the web to be monitored by a series of transmitters in a direction transverse to the direction of movement of the web. Radiation reflected from the web and the background is detected by a series of receivers. The measurement signals obtained in this way are passed to an electronic unit in which a quantity that characterizes the monitoring of the web is determined from the measurement signals. The approximate location of the edge, the edges, or of corresponding discontinuities of the web is detected "digitally" on the basis of the radiation beam of the radiation transmitter in whose area the greatest change takes place in the received radiation. The precise location of the edge or equivalent of the web to be monitored is determined analogously by comparing the amplitudes of the signals of the adjacent receivers placed closest to the last-mentioned radiation transmitter with one another.