To hold high telecentricity while securing a wide field angle and making an optical system small-sized, to secure sufficient back focus length, and further facilitate manufacturing.

A 1st lens 1 composed of a concave lens, a stop 3, and a 2nd lens 2 composed of a convex lens are arrayed in order from the object side and the 1st lens 1 and 2nd lens 2 meet the conditions of (1) 1.2f2<d2<1.6f2, (2) (f2)2/S≥2.2f1, and (3) 2f1< f1 <3f1, where f1 is the focal length of the 1st lens 1, 2 the focal length of the 2nd lens 2, f1 the focal length of the whole optical system, d2 the lens interval between the 1st lens 1 and 2nd lens 2, and S the distance from the front focus position of the 2nd lens 2 to the aperture stop 3.