PURPOSE: To eliminate on-axis chromatic aberration and magnification chromatic aberration and to correct spherical aberration and curvature of field in good balance by providing specific refractive index distribution in a radial direction perpendicular to an optical axis.

CONSTITUTION: This lens is a single lens of meniscus shape in which the refractive index distribution is provided in the RADIAL direction perpendicular to the optical axis, and an aperture diaphragm is provided at the recessed plane side of the lens, and a condition expressed in equation can be satisfied assuming the F number of an optical system as F and the angle of field of the lens as 2ω. The lens is composed in such a way that the plane on the object side of the lens is formed in a projecting plane, and the plane on an image side in a recessed plane, and it is used so as to provide the aperture diaphragm on the image side of the lens. Also, an image-forming plane is formed in a plane curved to a lens plane side, con-cretely, a cylindrical plane curved to the lens plane side.