Abstract: Apparatus for efficiently electrolyzing water comprising an electrolysis cell and gas-liquid separating mechanism, wherein the electrolysis cell is composed of a plurality of compartments, each of which has two plate electrodes with a membrane assembly sandwiched therebetween, the plate electrodes and the membrane assembly are spaced by a concavo-convex ion net and a concavo-convex nichel net respectively so that the spacing between the two electrodes is greatly reduced.