An infrared lens unit includes, in order from the object side, an aperture, a lens group, and a band-pass filter which passes a light beam of a prescribed wavelength band of the infrared region. The lens group further includes, in order from the object side, a first meniscus lens which has a positive power and is convex on the object side, a second meniscus lens which has a positive power and is convex on the image side, and a convex lens which has a positive power and is convex on the object side. The lens group further includes a near-telecentric state on the image side, wherein the angle of entry of the light beam into the band-pass filter is either 0° or close to 0°. Thus, it is possible to accurately pass a light beam of a prescribed wavelength band of the infrared region by the lens unit.