An optical system for an optical data recording and reproducing apparatus such as an optical video disc apparatus comprising a diode laser light source, a condenser lens disposed adjacent to the diode laser, a cylindrical lens assembly arranged to exhibit different magnifications to light beams emitted by the diode laser in the direction perpendicular to a junction plane thereof and in the direction parallel to the junction plane, and a concentrating lens for constricting the resulting light beam into a light spot of a small size for projection on a recording medium. Recording of information is attained by the constricted light beam of a substantially circular cross-section with a high recording density at a low energy consumption. When reproducing the recorded information, the constricted beam is laterally enlarged in the transverse direction of the information recording track thereby to assure generation of a signal utilized for tracking control.