The present invention is directed to an infrared zoom lens that consists merely of optical components of germanium so as to implement an optical system that is capable of reducing variation in brightness during varying a magnification rate and is quite bright and that facilitates compensating for aberration, especially spherical aberration that is generally hard to do, thereby producing a clear and vivid image. The infrared zoom lens comprises first to fourth groups of lens pieces arranged in series from the foremost position closest to the object; each of the lens groups having all the lens pieces made of germanium, and at least one of the lens groups consisting simply of a single lens piece.