PURPOSE: To maintain an etching speed within a specified range by making the potassium permanganate in an etching bath constant by electrolytic regeneration based on the measurement of the etching speed of a work in the etching bath.

CONSTITUTION: An etching soln. is sampled at every specified time from the etching bath 1 and the analysis values of concentrations of potassium permanganate, sodium hydroxide and potassium manganate are computed by a controller 3. An org. solvent of the same system as the system of the work is added from a stock tank 8 to an etching speed measuring cell 10 and the change rate from an initial concn. is computed by the controller 3, by which the etching speed is measured. The concn. of the potassium permanganate in the etching bath is managed to a specified concn. in accordance wit the result of the measurement of the etching speed.