Disclosed is a scanning optical system that includes a laser source for emitting a laser beam, a scanning deflector that deflects the laser beam, an imaging optical system that converges the scanning laser beam onto an object surface, first and second mirrors that bend the optical path of the scanning laser beam. The first and second mirrors are movable to adjust the optical path length between the deflector and the object surface for changing a width of the scanning range on the object surface. Since the optical path length is adjusted by moving the first and second mirror, which changes the width of the scanning range, correcting the size error of the printed image. When the size error of the printed image is detected, an operator moves the first and second mirrors to correct it.