To obtain a lens system in which the secondary spectrum of chromatic aberration is highly corrected with less optical components by specifying relation between a radial type GRIN lens whose refractive index is changed in a direction perpendicular to an optical axis and a diffraction type lens.

This achromatic lens includes the radial type graded index lens (radial type GRIN lens) whose refractive index is changed in the direction perpendicular to the optical axis and a diffraction type optical element (diffraction type lens), and a condition expressed by 0.1<θelgf<0.5 is satisfied. Provided that θelgf is the equivalent part dispersion ratio of the radial type GRIN lens. The diffraction type lens has a concentric circuit state pattern, and operates as the lens by the diffraction phenomenon of light, and has peculiar color dispersion characteristic in comparison with normal glass as Abbe number is -3.45. The achromatic lens is obtained by assembling the radial type GRIN lens and the diffraction type lens, so that the residual secondary spectrum of the chromatic aberration can be highly corrected.