PURPOSE: To provide a novel treating liquid for forming a stronger copper oxide coating not shown heretofore on the surface of copper, comprising an aqueous solution in which NH4OH and a water soluble metal hydroxide are dissolved respectively in a predetermined ratio.

CONSTITUTION: A treating liquid for forming a copper oxide coating on the surface of copper is prepared by dissolving 0.05W0.3mol of NH4OH per 1l of water and 0.1W2mol of a water soluble metal hydroxide (e. g.; KOH, NaOH) per 1mol of NH4OH, when the copper oxide coating is formed on the surface of copper by using this treating liquid, a means for immersing a copper body in the above described treating liquid or for dropping said treating liquid on the surface of the copper body is employed, but it is preferable to heat said treating liquid to 50W70°C in use because the formation of the copper oxide film is promoted. This method can be applied, for example, to a purpose for insulating a constitutional wire of the copper conductor of an electric cable.