To compensate chromatic aberration and monochromatic aberration by the fewest lens elements and to reduce cost by obtaining an infrared zoom lens in which an aspherical optical surface is not used but a diffraction optical element is used.

This lens is provided with four lens groups. The 1st lens group consists of two lens elements having surfaces 1 to 4. It is the lens group whose positive optical magnification is fixed and which consists of one silicon optical element and one germanium optical element to correct the chromatic aberration. The 2nd lens group has two lens elements having surfaces 5 to 8 being variable elements having linear zoom motion. The 3rd lens group is a single lens element having surfaces 9 and 10, has negative refractive power and works as a compensator. The 4th lens group behind a diaphragm 11 consists of four lens elements having surfaces 12 to 19. The 4th lens group is moved to perform focusing when a subject position is changed from infinity to a finite distance from the lens.