PURPOSE: To raise the detecting accuracy of bad printing, by such constitution as to arrange plural light sources enbloc vertically to the moving direction of printing and in a line and to receive the reflected light by the photoelectric element through slits and cylindrical lenses.

CONSTITUTION: The light projecting device 110 and the light receiving device 120 are arranged in the case 103 forming 1 channel, and each channel is fitted to the fitting hole 104, and by means of inserting the cover, the optical detecting device of n channels in total is formed. This device is arranged vertically and in line against the moving direction of the printed matter as shown by an arrow. The light projecting device 110 furnishes lamp 112. condenser lens 113, and projection lens 114, and the projected light is reflected by the plane mirror 105 and illuminates the printed matter 1 via slit 107, converging lens 121, slit 122, cylindrical lens 123. Thus, the detecting accuracy of bad printing is raised.