This paper reports the design and analysis of the infrared optical system for automotive thermal cameras. Through optimization, we have obtained an FIR (Far Infrared) optical system with a focal length of 6.2 mm, f/1.1, and its field of view is 61.6°×45.5°. The system is consist of two lenses including three aspheric surfaces and one diffractive-aspheric surface, which were used to balance the higher-order aberrations. In addition, to make the athermalized optical system, As40Se60 glass with the lowest thermo-optic coefficient (dn/dT) among the commercialized chalcogen glasses is applied as a lens material, and each lens is designed so that defocus occurs in opposite directions depending on temperature. In conclusion, we suppose the optical design is suitable for an automotive thermal cameras.