A far-infrared camera includes three lenses formed of ZnS. A first lens is a biconvex lens, a second lens is a negative meniscus or biconcave lens, and a third lens is a positive meniscus lens. A diffraction surface is formed in either surface of a lens. When a total focal distance f of the lens system is 10 mm to 30 mm and a focal distance f12 of the first and second lenses is 20 mm to 70 mm, 1<=f12/f<=3.