PURPOSE: To secure a small lens system, being high performance at a wide field angle and bright, with a triplet lens of 3-group/three sheet constitution, by forming at least one surface of both second and third lenses into aspheric surface, and making it satisfy a specific condition.

CONSTITUTION: This triplet lens consists of a first lens L1 or positive meniscus lens directing a convex to the body side, a second lens L2 or biconcave lens, a third lens L3 or biconvex lens, and a stop S. In this constitution, at least one surface out of these second and third lenses L2 and L3 is formed into an aspheric surface, making it satisfy a conditional expression of 1×10-5<x1/f<1×10-2, 0.5<r5/f<1.1. Where, x1 is slippage, from the reference spherical surface, of the aspheric surface in at least one effective maximum diameter out of the aspheric surface, r3 is a radius of curvature of the body side of the third lens L3, and f is a focal distance of the whole system. In this connection, when this aspheric surface is used afterward the third surface, its effect grows larger.