An optical scanning system for a printer, having an optical scanning unit (30) for scanning light onto a circulating photoreceptor web (14); a photodetector (18) for receiving scanning light emitted from the optical scanning unit. A defective edge area detector (42) is provided for comparing pulse signals output in sequence from the photodetector (18) to determine whether the edge of the photoreceptor web has a defect, and outputting an edge defect signal if a defective area is detected at the edge of the photoreceptor web. An estimated pulse width calculator (42) is used to provide a predetermined estimated pulse width, and updates the estimated pulse width for the next line image scanning using the signal from the defective edge area detector, the predetermined estimated pulse width, and the pulse signal from the photodetector. A line scanning synchronous signal generator (43) generates a line scanning synchronous signal in synchronicity with the predetermined estimated pulse width; and an optical scanning unit driver is provided to drive the optical scanning unit such that the optical scanning unit emits the light onto image information synchronized with the line scanning synchronous signal. By the optical scanning system and a method for adjusting the start point of image scanning, errors in writing an image can be suppressed, which is caused by a defect at the edge of the photoreceptor web.