To provide a graded index type signal lens composed of one lens and having high performance at a low cost by making the lens has a constitution having a specified distribution of refractive index and satisfying a specified condition.

This leans is composed of a radial type graded index lens 1 whose both surfaces are plane and have a distribution of refractive index represented by n(r)=No+N1r2+N2r4+Sv, a diaphragm 2 is located on the entrance plane of the graded index lens 1 and an image plane 3 is located on the outside of the graded index lens 1. The graded index lens 1 satisfys four conditions: 1<t/f 3.5,-0.2<f4.N2<0.2, -2<d/t<0.5, -0.015<1/V1<0.015. In the relations, f: the focal distance of the lens, t: the thickness of the lens, d: distance from the surface of the lens on the object side to a brightness diaphragm in the direction of optical axis, n(r): refractive index of the graded index lens 1 at distance r in the direction being coincident with the optical axis, No: refractive index on the optical axis, N1 N2: second order and fourth order coefficients of refractive indices and V1: coefficient representing the Abbe number of a medium.