PURPOSE: To prevent the discoloration of copper on the silver peeled surface of a silver plated copper base material by using a treating liquid which contains 5-methyl-1H-benzotriazole, etc., and is made acidic by an inorg. acid, etc., for preventing the discoloration of the silver peeled surface.

CONSTITUTION: Silver plating is further applied on the surface of the base material formed by plating copper or copper alloy on copper or copper alloy or metallic material and thereafter, the silver sticking to the part where the silver plating is not necessary is peeled. This base material is immersed into a treating liquid of the copper discoloration preventive which contains one or ≥2 kinds selected from the 5-methyl-1H-benzotriazole, 5,6-dimethyl-1H- benzotriazole, 2-mercaptopyridine and is made acidic by the inorg. acid and/or org. acid. The discoloration of the copper of the silver peeled surface is prevented in this way and the reliability of electronic parts materials is enhanced by using this liquid for these materials.