PURPOSE: To reduce a variation of a focal distance by adopting an aspherical surface for each one face of second (=1 fifth) lens, and a third (=a fourth) lens, and allowing it to satisfy a specific condition.

CONSTITUTION: An aspherical surface is adopted for each one face of the second (the fifth) lens, and the third (the fourth) lens, and allowed to satisfy a condition of -1.0<f2S/f3S<-0.9. In this regard, f2S and f3S denote a focal distance of the second (=the fifth) lens at a temperature of a reference state, and a focal distance of the third (=the fourth) lens at a temperature of a reference state, respectively. That is, by using a plastic material a convex lens and a concave lens, a variation of a focal distance at the time when a radius of curvature, thickness, and a refractive index have been varied due to a temperature change offset by the convex and concave lenses. In such a way, a variation of a focal distance can be suppressed small as the whole lens.