A preform is prepared to comprise a step index structure consisting of a core and a clad both made of metal halogenides or arsenic-selenium glass sparingly capable of transmitting infrared light of long wavelength and a layer of lubricant applied to the outer surface of the clad. The preform is set inside a metal cylinder. By pulling this metal cylinder through a plurality of dies containing orifices of successively decreased diameters at a temperature below the melting point, the preform can be elongated into an optical fiber of a prescribed diameter. The optical fiber thus produced is capable of transmitting infrared light of high power with a small loss. Protected and reinforced by the metal cylinder jacket, this fiber enjoys high mechanical strength.