A continuous electrolytic treatment can be applied to a metal strip by the method which comprises the steps of (1) passing a metal strip through a narrow treating space formed between horizontal upper and lower electrode devices, each having at least one insoluble electrode, whereby the treating space is divided into two gaps by the metal strip; (2) feeding an electrolytic treating liquid to the gaps through slits each formed in the middle portion of the electrode device in such a manner that the slit horizontally extends across the electrode device at right angles to the direction of passage of the metal strip and directed vertically toward the metal strip, whereby each stream of the treating liquid can be divided into two opposite flows in the gap; and (3) applying an electric current between each electrode and the metal strip.