PURPOSE: To obtain an optical system which has the wavefront satisfactorily corrected to be superior in massproductivity, by satisfying a specific condition among the refractive index, the focal length, the numerical aperture of a spherical lens and the interval between the divergance origin of a divergent luminous flux and the spherical lens end face.

CONSTITUTION: The collimating optical system which converts the divergent luminous flux to parallel rays of a luminous through the spherical lens satisfies the consition of an inequality where (n), (f), NA, and lopt are the refractive index of the spherical lens, the focal length of the spherical lens, and the numerical sperture of the spherical lens, and the interval between the divergence origin of the divergent luminous flux and the spherical lens end face respectively. This optical system consists of only two optical members, namely, a spherical lens 1 and a stop 3, and only the spherical lens 1 is used as lenses, and the stop 3 is arranged near the luminous flux exit-side end face of the spherical lens 1 and practically determines NA of the optical system. Since the spherical lens is used as an essential element, the constitution is simplified and the cost is reduced. Said condition is satisfied to reduce the occurrent of aberrations, and the colimating optical system has a wide field angle and a high performance is obtained.