An optical system for recording and reading information on an optical information medium such as a video disk, a mini disk, a magneto-optical disk, and a phase change disk. The optical system includes: a light source for irradiating the optical information medium with a light beam through a coupling lens and a objective lens; the coupling lens which is a spherical single lens; and the objective lens which is a double-sided aspherical lens and provided movable along an optical axis of the light source; in which the light source is located between the coupling lens and a focal point, at a side of the light source, of the coupling lens. The optical system defines the number of a numerical aperture on the side of the optical information medium, and a lateral magnification ratio of the optical system.