PURPOSE: To remove strain by a simplified means and to improve transmission characteristics by electrifying and heating various metallic materials for signal transmission.

CONSTITUTION: At the time of performing stress-relief annealing for wires, foil materials, coated materials, etc., made of high-purity metal, such as Cu and Al, heat treatment is carried out by means of electrification. A temp. of about 50W120°C suffices for electric heating. Since an electric current corresponding merely to the size of an object is allowed to flow and self- exothermicity is applied in the above method, the presence of a casing is not an obstacle but is, on the contrary, suitable for shutting off from the surrounding air. Moreover, by controlling the amount of electrification, precise heat treatment can be attained uniformly as a whole.