A laser scanning optical system including a rotatable polygonal mirror for scanning the photosensitive surface with the laser beam in a scanning direction, a lens unit for converging the laser beam reflected by the polygonal mirror on the photosensitive surface, a first reflection mirror for reflecting the laser beam passed though the lens unit to the photosensitive surface, a second reflection mirror for reflecting the laser beam reflected by a part of the first reflection mirror, a sensor arranged to receive the laser beam which is reflected by the second reflection mirror and has passed through a specific position equivalent to the photosensitive surface for generating a synchronizing signal for commanding start of modulation of the laser beam, and first and second lens elements both interposed between the specific position and the sensor so as to keep a conjugate condition therebetween. The first lens element has a positive power only in the direction perpendicular to the scanning direction and provides a magnification smaller than 1 in the direction perpendicular to the scanning direction, while the second lens element has a positive power only in the scanning direction and provides a magnification greater than 1 in the scanning direction.