A multiplexing optical system includes collimator lenses, a first condensing lens, a second condensing lens and an optical fiber. The collimator lenses collimate divergent laser beams that have been emitted from semiconductor lasers. The first condensing lens condenses laser beams transmitted through the collimator lenses in only one of a plane including the stripe width direction of the semiconductor lasers and a plane including a direction perpendicular to the stripe width direction. The second condensing lens condenses laser beams transmitted through the first condensing lens. The optical fiber is arranged in such a manner that the condensed laser beams enter the optical fiber. In the multiplexing optical system, an anamorphic lens is used as the second condensing lens, and the anamorphic lens condenses the laser beams in the two planes in cooperation with the first condensing lens.