To provide an optical system reducing the number of constitutional lenses, considering the difficulty and cost of manufacturing of a low material and capable of attaining sufficiently enough performance and miniaturization by the use of lenses of a small refractive index difference by including a specific refractive index distribution type lens in a pre-group and including a positive lens in a post group.

A refractive index distribution type lens having a refractive index distribution expressed by n(r)=n0d+n1dr2+n2dr4+n3dr6+... and satisfying condition of 0.1<n1d×t2<0.8 is included in a positive pre-group, and a negative lens adjacent to an iris and a positive lens on its image side are included in a positive post group. Here, nd(r) is the refractive index of a (d) line at a position of radius (r) from the optical axis n0d is the refractive index of a (d) line on the optical axis, n1d, n2d, n3d,... are the second, 4th and 6th order refractive index distribution coefficients for the (d) line, and (t) is the thickness of the refractive index distribution type lens on the optical axis. Consequency on optical system reduced at the number of lenses, allowed to be easily manufactured at a low cost and having excellent optical performance can be obtained.