To enlarge the numeral aperture without increasing aberration and attain miniaturization by combining a specific refraction factor distribution lens with a single homogeneous plano-convex lens.

A single refraction factor distribution lens 1 having refraction factor distribution in the radial direction is combined with a single homogeneous plano-convex lens 2 having uniform refraction factor. The refraction factor distribution lens 1 has a plane or convex spherical lens surface, and when the refraction distribution is expressed as follows; n(r2)=n02.(1-(g.r)2+h4(g.r)4+ h6(g.r)6+h8(g.r)8+..., inequalities are expressed as 1.45≤n0≤1.80, 0.45≤n0.g.r0<0.90, where r is a distance from an optical axis, n(r) is refraction factor on the position of distance r, n0 is refraction factor on the optical axis, r0 is radius of the refraction factor distribution lens, and g, h4, h6... are refraction factor distrubution factors. The optical axis of the homogeneous convex lens 2 is arranged on the same straight line with the optical axis of the refraction factor distribution lens, and this optical system uses a refraction factor distribution lens satisfying the condition of 2.2<n.