Electrolyzer for catholyte production comprises an inner hollow cylindrical anode, an outer cylindrical cathode, and a diaphragm interposed between them. The length of the working part of the anode is at most 2 to 6 times the outer diameter of the anode. The inner hollow anode is made of one or two sections, the sections being connected to each other by a flow dielectric cylindrical sleeve having a diameter not larger than the outer diameter of the anode. The inner hollow anode has openings for introduction of water into inner cavity of the anode and openings for discharge of water at opposite ends of diameters of the anode lid. The electrolyzer for catholyte production operates in a horizontal position because outlet openings of the anode lid are located at the ends of the diameter of the anode lid, close to the outlet openings of the electrolyzer lid facing vertically upwards.