following condition is satisfied 0 lt u03b2 u2266 2 where u03b2 is the imaging lateral magnification of the optical system in the sub scanning direction application_number US 39454289 A description fig4 a and 4b are schematic views of essential portions of the first embodiment taken in cross section perpendicular to the plane of deflection and in a plane parallel to the plane of deflection respectively the plane of deflection refers to the light beam plane formed with the lapse of time by a light beam deflected by the deflecting reflecting surface of a deflector the main scanning direction refers to the direction in which the light beam deflected by the deflecting reflecting surface of the deflector scans a surface to be scanned and the sub scanning direction refers to the direction perpendicular to the main scanning direction in fig4 a and 4b the reference numeral 1 designates a semiconductor laser having two light emitting portions 1a and 1b disposed at an interval x 1 in the sub scanning direction on a common monolithic substrate the reference numeral 2 denotes a collimator lens which collimates two laser beams emitted from the two light emitting portions 1a and 1b at predetermined angles respectively the reference numeral 3 designates a stop which shapes the light beams from the collimator lens 2 to make the spot diameters of the laser beams on a surface 8 to be scanned which will be appropriate as described later the reference character 4a schematically shows one deflecting reflecting surface of a rotatable polygonal mirror as a light deflector and the laser beams are reflected and deflected by the deflecting reflecting surface 4a the reference numeral 5 denotes a scanning lens having the f u03b8 characteristic.