PURPOSE: To provide a producing method of an electrolytic ionized water which is safe, low in power consumption unit and excellent in detergency for a silicon wafer having high oxidation and reduction potential.

CONSTITUTION: In an electrolytic cell provided with an electrode chamber formed by using a cation exchange membrane as a diaphragm, the electrolytic ionized water is produced by supplying an aq. solution containing a quaternary ammonium compound to an anode chamber or both of the anode chamber and the cathode chamber to electrolyze. As the quaternary ammonium compound, at least one of a tetraalkyl ammonium compound and a tetraalkyl alkanol compound can be used.