A web width adjusting device comprises a pressure force applying unit for applying pressure force to one side surface of a paper web which is successively traveled through two or more printing sections. Further the web width adjusting device may also include a counter pressure applying unit to apply counter pressure to the other side surface of the paper web. The pressure applying unit (and the counter pressure applying unit) deformes the paper web in a wavy surface which causes the width of the paper web to be shortened. The adjusting factors for the web width are controlled by automatic control system which detects traveling condition of the paper web, the generation of shears in printing, etc.