An image forming apparatus containing a divisional scanning type optical scanner. In the apparatus, if displacement of one line in a subscanning direction occurs in a joint of image formation areas, light sources are controlled so as to modulate and emit light beams based on image data of the scanning line shifted one line at the same point of time. If displacement of 0.5 pixels in a main scanning direction occurs in a joint of image formation areas, the emission start timing of one light beam is shifted by the scanning time as long as 0.5 pixels with respect to the emission start timing of the other light beam, thereby correcting the displacement amount in the main scanning direction.