To provide a small-sized image forming optical system using low- dispersion refractive index distribution material which can be easily manufactured, and capable of satisfactorily correcting various kinds of aberration with a small number of about three lenses, and covering the extent of a lens from a standard lens whose viewing angle is about 50 to 60° to a wide angle lens.

The optical system is a lens system constituted of three groups, and in order from an object side, the system is constituted of a 1st group G1 whose refractive power is negative, a 2nd group G2 which is constituted of one meniscus lens whose concave face faces an image side and a 3rd group G3 including at least one refractive index distribution lens and whose refractive power is positive and the refractive index distribution lens is the radial type one, and the 1st group G1 is constituted of one concave lens, and the 3rd group G3 is constituted of one radial type refractive index distribution lens, and by which -0.005<1/V1<0.01 is satisfied, provided that V1 denotes Abbe's number of the radial type refractive index distribution lens on a d-line.