PURPOSE: To obtain a lens in which an aberration variation extending from a wide angle end to a telephoto end scarcely occurs, and also, an image surface extending from the center part of a screen to the peripheral part is flat, by constituting a zoom lens of a four-group zoom system, and using a suitable refractive index distribution type lens for said system.

CONSTITUTION: An axial type refractive index distribution type lens having refractive index distribution in the optical axis direction is used in a lens group, an aberration remaining quantity of the lens group is reduced and an aberration variation at the time of zooming is suppressed, and an extremely satisfactory image forming state is obtained. That is to say, when fW, f4, β4W, and β4T are assumed as a focal distance of the whole system in the wide end, a focal distance of the fourth lens group, the magnification in the wide angle end, and the magnification in the telephoto end, respectively, the fourth lens group is allowed to satisfy the expressions I, II. Also, a third lens group is allowed to satisfy the expressions III, IV, when β3W and β3T are assumed as the magnifications in the wide angle end and the telephoto end, respectively, and allowed to satisfy the expressions V, VI, when nA and (n) are assumed as the maximum refractive index difference between an object side surface term and an image side surface term on the optical axis, and a refractive index in the object side surface term, respectively.