PURPOSE: To obtain a lens system in which a bend of a curvature of an image is corrected by constituting an optical system consisting of the first negative lens by a composition of a joint of positive and negative lenses, the second and the third positive meniscus lenses, and the fourth negative lens of a joint of the nagative and positive lenses in order from an object side so as to satisfy prescribed conditions.

CONSTITUTION: A diaphragm S is provided between the second lens L2 and the third lens L3 of an optical system consisting of the firstWthe fourth lenses L1WL4. This symmetrical type lens is constituted so as to satisfy various conditions of an expression I W an expression V in said optical system. In this regard, in the expressions, D:d3:(f): l:rl:N1:N2: and r4 denote a center thickness of the first lens L1, an air interval between the first and the second lenses L1, L2, a focal distance of the whole system, the overall length of a lens system, a radius of curvature of the surface of the opposite side against the diaphragm S of the first lens L1, a refractive index against a (d) line (λ= 588nm) of a positive lens for constituting the first lens L1, a refractive index against a (d) line of a negative lens for constituting the first lens L1, and a radius of curvature of the surface of the opposite side against the diaphragm S of the second lens L2, respectively.