Thermoplastic synthetic polymer fibers having powder of metal attached to the surface of the fibers are subjected to a treatment for increasing the oxidation number of said metal from zero to a plus value, that is, oxidation in a broad sense. The resulting conductive synthetic fibers support a layer of a compound of the metal, i.e. an oxidized product of the metal in a broad sense, firmly and undetchably thereon, and have as low an electric resistivity as 10.sup.5 .OMEGA./cm or less. The conductive fibers can be blended to ordinary synthetic fibers to improve their antistatic property, or can be substituted for expensive metal fibers. The thermoplastic synthetic polymer fibers are sheath-core fibers wherein the sheath has a lower melting point than the core.