PURPOSE: To increase an electric discharge machining speed and perform electric discharge machining at high tension of a wire, by heating a Cu-Zn system alloy wire containing specific % Zn in an atmosphere, almost not oxidizing Cu but oxidizing Zn, to be elongated after almost not oxidizing Cu but oxidizing Zn, to be elongated after a film mainly composed of ZnO is formed on a surface of the wire.

CONSTITUTION: A Cu-Zn system alloy wire consists of a Cu-Zn alloy containing 20W40% Zn or an alloy adding to this alloy an addition element of Aλ, Si, Zr, Ti, etc. improving tensile strength particularly high temperature strength, and the Cu-Zn system alloy wire, for forming on its surface a desired film, applies heat treatment in an atmosphere almost not oxidizing Cu but oxidizing Zn. The atmosphere is such that oxidation partial pressure is 1/105W1/10 atm generally at 450°C or more and a heating time is generally required 45min or more at 500°C or more and 1/104W1/102 atm. And if the oxidation partial pressure exceeds 1/10 atm, an electrode wire having a high electric discharge machining sped is not obtained, while a film of desired thickness is not easily formed for a heating time below 45min, here an elongated wire, after it forms a film, is hardened by oxidation treatment, increasing wire tension at electric discharge machining time.