An antireflection film composed of a layer of arsenic triselenide having an optical thickness in the range between 2.518 and 2.783 mu m, a layer of potassium chloride having an optical thickness in the range between 1.151 and 1.272 mu m and a layer of arsenic triselenide having an optical thickness in the range between 0.577 and 0.749 mu m which are successively formed on a member of infrared radiation transmitting material formed of a mixture of thalium iodide and thalium bromide. The member provided with the three-layer antireflection film is used in an atmosphere of protective gas, such as dry air or dry N2 gas.
