PURPOSE: To continuously maintain the etching capability of the etching solution linking with an etching device in operation.

CONSTITUTION: An electrolytic cell having an anode chamber 5a which has an anode 8a made of a porous material and also has an entrance 11a and an exit 12a across the anode 8a, a cathode chamber 6a which has a cathode 14a and an electrolyte of an alkali solution, and an ion exchange membrane 7 which partitions those anode chamber 5a and cathode chamber 6a is used and the etchant which contains ferrous ions and ferric ions is passed through the porous anode 8a with large surface area to continuously recover and maintain the capability without producing secondary products while expediting anode reaction. At the same time, even if ferrous ions and ferric ions of the etching solution partially migrate to the cathode chamber 6a through the alkali solution, they are precipitated as hydrides and metal is prevented from being deposited on the cathode surface to carry on the cathode reaction, thereby continuously maintaining the capability with efficiency.