Unevenness of light quantity caused by angle dependence of transmittance is decreased, and a light beam is shaped. Therefore, a light source 3 which emits the polarized light beam having a long shape and an aperture 10 which narrows the light beam emitted from the light source 3 are provided. An opening 10a of the aperture 10 is formed in a long hole shape. A line of apsides direction of the opening 10a of the aperture 10 is set to a minor axis direction of the light beam from the light source 3, and the line of apsides direction of the opening 10a of the aperture 10 is formed so as to extend to an outside of the area defined by 1/e.sup.2 for the peak intensity of the laser beam. Polarized directions of the light beams emitted from the light sources 3 are arranged so as to have a symmetric relation or a substantially symmetric relation with respect to a straight line extending toward a sub-scanning direction.