A beam-shaping optical system includes a light source, a plurality of plates, and a cylindrical lens disposed between the light source and the plurality of plates, wherein the plurality of plates and the cylindrical lens have an optical feature in which the light output from the light source is beam-shaped to have a desired shape in the cross-section of the light beam. An optical pickup using the beam-shaped light which is diverged by the optical system can obtain convergence light reflected from an optical storage medium. Since the convergence light transmitted through only one plate has astigmatism as it is, a separate lens is not used for light detection and uses a photo detector adapted for employing an astigmatic method, to perform a focussing servo.