In an electrolytic treatment of alkali halide for the manufacture of alkali hydroxide, a three-chamber type reaction vessel assembly is used so as to provide an anodic chamber, an intermediate chamber and a cathodic chamber arranged in succession one after another and mutually separated by means of anodic (i.e., cationic selective) ion exchange membranes. From the intermediate chamber, low concentration caustic alkali solution is obtained, while from the cathodic chamber, high concentration caustic alkali solution is obtained. In this electrolytic process, the system is operated with the cathodic chamber supplied with no water or aqueous content. The high concentration caustic alkali solution is made from water and alkali metal ions conveyed from the intermediate chamber to the cathodic chamber and exclusively and osmotically passed through the anodic ion exchange membrane. The polymers or copolymers of the anodic ion exhange membrane include sulfonic acid, carboxylic acid and/or phenolic radicals. As the method for the introduction of these radicals, formation of corresponding copolymers may be utilized in the most recommended way. As an example, the membrane may comprise a fluorinated polymer having a cyclic ring structure unit and pendant type sulfonic acid radicals.