An optical system includes a plurality of separate light sources with a number M of the light sources arranged in a first direction and a number N of the light sources arranged in a second direction. Each of the light sources has an asymmetric light emission area. The light sources are arranged so that a light beam passing a collimator optical element has a beam width in the first direction greater than a beam width in the second direction. An anamorphic optical element is arranged to have a beam-size reduction rate in the first direction greater than a beam-size reduction rate in the second direction and converts the plurality of light beams from the light sources into a light beam having a symmetric beam cross-section. A light-condensing optical element condenses the light beam having the symmetric beam cross-section into a light receiving element, and the anamorphic optical element includes an array of a number of prisms integrally arranged.