The present invention relates to an electrolyzer for treating an object liquid. A cylindrical anode plate is formed as an outer wall of said electrolyzer and a cylindrical cathode plate is coaxially arranged at a position close to an inner surface of said anode plate. An ion exchange membrane is coaxially arranged between both electrodes to form an isolating chamber for a selective electrophoretic separation and removal and a concentration. A liquid, which has been subjected to an aimed treatment in said isolating chamber, is uniformly flown out from a whole circumference through a circumferential passage provided in an upper portion of an electrode portion. Thus, anions, which have acted upon metal cations, are dissociated in said isolating chamber to selectively separate merely cations. In addition, an improvement of the anode brought into contact with said object liquid in durability, a simplification of electrode surfaces in repair and processing, a uniformation of a current density, a handling of the liquid under the conditions corresponding to an operating object of the electrolyzer and the like can be achieved.