PURPOSE: To provide a lens system consisting of minimum number of optical elements without necessitating the negative lens of zinc selenide, etc., for color correction in an infrared area.

CONSTITUTION: This infrared continuously zooming telescope, namely afocal lens system, performs color correction by using a diffractive face. The lens system is provided with an objective lens 1, an eyepiece 9, a plurality of lenses 3, 5 and 7 provided between the lenses 1 and 9 to give a zooming function and a diffractive pattern provided at one face of the objective lens 1, desirably integrated with its inner surface, to give color correction with the prescribed wavelength of light passing through the objective lens. It is desirable that all the lenses are germanium. A diffractive pattern is provided with a plurality of ring-formed steps with intervals, each ring is coaxial with the center of the objective lens and the height of each step is equal to a prescribed wavelength. The plurality of ring-formed steps are formed according to an already known expression.