PURPOSE: To remove rust generated on the copper foil part or the like of a printed wiring boad without the disconnection of printed wiring by treating the rust by an aq. soln. contg. ammonium persulfate, concd. sulfuric acid and a phosphoric ester series surfactant.

CONSTITUTION: In the case rust is generated on the copper foil of a printed wiring material obtd. by forming printed wiring constituted of copper foil or the like on an insulating substrate, by using an aq. soln. contg. 10 to 150g/l ammonium persulfate, 10 to 100g/l concd. sulfuric acid and 0.0001 to 0.05g/l). phosphoric ester series surfactant, the rust on the copper foil can easily and completely be removed without the disconnection of the printed wiring.