Abstract: An infrared optical system, which can obtain excellent image even when used under a severe condition, is easily and cheaply produced by molding. The infrared optical system, is provided with a configuration of 2 groups 2 lenses having the first lens L1 and the second lens L2 sequentially from the object side, the first lens is a convex aspheric lens of which convex surface protrudes toward the object side, and the second lens is a convex meniscus lens of which convex surface protrudes toward the object side, the infrared optical system is formed by using chalcogenide glass such that the extreme values of the surface shape of the first lens L1 and the second lens L2 are not formed at the lens surface including the outer portion of the lens effective diameter.