PURPOSE: To reduce a number of lenses. to make a half viewing angle a wide angle of 20°, to excellently compensate chromatic aberration in a wide wavelength region and to increase MTF for a high special frequency by composing the lens of three-group four-lenses.

CONSTITUTION: A first lens 11 arranged on the object side is bi-convex lens, a second lens 12 on the image side is bi-concave lens, a third and a fourth lenses 13, 14 arranged on the image side of the second lens 12 and mutually joined are bi-convex and bi-concave lenses, respectively, and the surface of the first lens 11 on the object side is an aspherical surface. By representing the composite focal distance of the whole system for (e) line by (f), the focal distances of the first and the second groups for (e) line by f1, f2 the refractive index and the Abbe number of the material of the jth lens for (d) line by nj, νj (J=1-4), respectively, the conditions: 0.4f<f1< 0.5f, -0.3<f2<-0.4f, 0.1<{(n1+n3)-(n2+n4)}/2<0.2, 9.0<{(ν1+ν3)-(ν2+ν4)}/2<10.5 are satisfied.