PURPOSE: To prevent breakdown of a core material and to form a stable composite material, by covering the circumference of the core material with a sheathing material being softer than the core material, and performing the twist- working to a composite material to be worked, in the intermediate stage of the diameter elongation and reduction working.

CONSTITUTION: Copper, iron, steel, nickel, or their alloy, or a super-conductive material is used as a core material 1, and this core material 1 is covered with aluminum, copper, nickel, or their alloy, as a sheathing material 2. When this composite material is subjected to diameter reduction working, when the twist- working is performed in the intermediate stage, compressive stress and tensile stress work on the core material 1 and the sheathing material 2, respectively, in a stress distribution B by the twist-working, it becomes opposite to a stress distribution A by the diameter reduction working, and when the stress distributions A, B are synthesized, each internal stress becomes very small like a stress distribution C, or on the contrary, the compressive stress works on the core material 1 at a twisting pitch, and even if the composite material is subjected to the diameter reduction working, no breakdown of the core material 1 occurs.