A light exposure device for a color copier has a plurality of laser light sources for emitting laser beams on the basis of image signals of separated colors. These laser beams are continuously deflected by a polygon mirror to allow the surfaces of the photosensitive drums which are provided correspondingly to the respective light sources to be scanned with the laser beams. Through the scanning of the laser beams by the polygon mirror, electrostatic latent images corresponding to the respective colors are formed on the surfaces of the photosensitive drums. At this time, a portion of a laser beam emitted from a reference light source is received by a light receiving element and, by doing so, a sync signal is obtained. Based on the sync signal it is possible to control write timings of the laser beams emitted from the laser beam sources. Further, register patterns output onto a transfer belt at that time are read out by an image sensor so that a shift relative to the patterns is detected and, by doing so, the write timings of the laser beams are so corrected as to bring the relative shift to zero.