An optical system includes an objective lens; and a plastic coupling lens to change a divergent angle of a light flux emitted from a light source and is structured such that the light flux emitted from the light source passes through both of the coupling lens and the objective lens when the light flux is converged onto an optical information recording medium and the reflected light flux reflected from the optical information recording medium passes through the objective lens without passing through the coupling lens when the reflected light flux is detected by a photo-detector. A ring-shaped diffractive structure is formed on at least one surface of the coupling lens and the diffractive structure generates an under-spherical aberration when a light flux having a longer wavelength comes in, and the coupling lens has a positive refractive power as a whole.