the object of the invention is to accomplish surface treatment such as cleaning , etching or after - treatment of parts or semi - processed articles for the manufacture of industrial or medical products without raising the problem of environmental pollution and without damaging or contaminating the treated articles . water is electrolyzed in a tank which is partitioned by a porous membrane into an anode chamber and a cathode chamber to prepare a fresh anodic water containing h + ion in the anode chamber and a fresh cathodic water containing oh - ion in the cathode chamber . the fresh anodic water and the fresh cathodic water are separately discharged from the electrolysis tank , and selected one of the anodic water and the cathodic water is brought into contact with the object of wet treatment . the efficiency of electrolysis of water can be enhanced by adding a small amount of an electrolysis aid such as carbon dioxide gas or ammonium acetate to water and / or by irradiating water under electrolysis with an electromagnetic wave having an wavelength not longer than 400 nm or not shorter than 3000 nm .