Provided is an infrared optical system including: three lenses of a first lens, a second lens, and a third lens that are provided from an object side to an image-surface side, the first and third lenses being each configured as a spherical lens made of an inorganic material and having a positive refractive power, the second lens being configured as a meniscus lens made of a resin material and having aspherical surfaces; and an aperture stop that is provided between any two of the first to third lenses.