This infrared optical system using the wavelength range of 3 to 5 μm comprises, in the order from an object side, a first lens having a negative power and formed with a convex meniscus toward an object, a second lens that is convex on both sides, and a cold aperture. A bandpass filter is provided between the second lens and the image surface. The first and second lenses are configured with silicon or germanium with each provided with an aspheric surface at least on one side. The infrared optical system satisfies the conditional expressions: 0.1 μm