An infrared optical system includes: a first optical element, a second optical element, a third optical element, and a fourth optical element arranged along a direction from an object side toward an image plane side, wherein the fourth optical element has positive refracting power, an aperture is provided between the second optical element and the third optical element, the first and fourth optical elements are made of any one of silicon (Si), chalcogenide glass, and zinc sulfide (ZnS), and the second and third optical elements are made of a resin material, and at least one of a surface facing the object and a surface facing the image plane of each of the second and third optical elements is an aspheric surface.