PURPOSE: To accelerate the electrolytic oxidation reaction of an anode and to continuously restore and maintain the capacity of a liq. etchant by linking an electrolytic cell with an etching device in operation.

CONSTITUTION: An electrolyte-permeable anode 8 consisting of a porous material is provided in an anode compartment 5, an electrolytic cell is divided by a proton permselective cation-exchange membrane 7 into the anode compartment 5 and cathode compartment 6, and a liq. etchant contg. ferrous ion or ferric ion or cuprous ion and cupric ion is continuously supplied from the inlet of the anode compartment 5, permeated through the anode 8 and oxidized. Consequently, the area of the electrode plate is reduced, and the capacity of the etchant is continously restored and maintained.