An infrared zoom lens comprises, in order from the subject end to the image end, a positive power first lens group having lens elements less than three, a negative power second lens group having lens elements less than three, a third lens group having a single meniscus lens element with a concave subject side surface, a fourth lens group having a single convex lens element, and a positive power fifth lens group having at least four lens elements which include a convex meniscus lens with a concave image side surface facing directly to an image plane, the second and third lens groups being axially movable in predetermined relation relative to each other and relative to the first, fourth and fifth lens groups which are stationary to vary the zoom ratio of the infrared zoom lens and form a sharp image on the image plane, and satisfies the following conditions:______________________________________ 1.00 f2/ft 0.35