Method and apparatus for producing deionized water

An apparatus for producing deionized water comprising an electrodialyzer having cation exchange membranes and anion exchange membranes alternately arranged between a cathode and an anode to form demineralizing compartments and concentrating compartments, and ion exchangers accommodated in the demineralizing compartments, wherein a flexible spacer is arranged within each concentrating compartment to maintain the thickness of the compartment and to exert a pressure of from 0.1 to 20 kg/cm.sup.2 between the ion exchangers accommodated in the demineralizing compartments and the cation exchange membranes and anion exchange membranes defining the demineralizing compartments and

• wherein the pressure exerted between the ion exchangers accommodated in the demineralizing compartments and the cation exchange membranes and anion exchange membranes is such that the ion exchangers to be accommodated in the demineralizing compartments are converted to a form having their volume reduced smaller than the volume of their regenerated form and then packed in the demineralizing compartments in an amount such that the volume of the regenerated form of the ion exchanger in a free state becomes from 103 to 170% relative to the volume of the demineralizing compartments, followed by supplying water and conducting an electric current to let the ion exchangers expand to increase the volume thereby to increase the pressure.