A method of purifying cooling water which requires the lowest maintenance and management cost without the need for a cumbersome cleaning operation for removing scale in an electrolytic purifying vessel by taking out the electrodes from the electrolytic purifying vessel, and an apparatus therefor are provided. In the method of purifying circulating cooling water by applying a DC voltage across opposing electrodes while flowing water to be treated therebetween, so that ions in the water are electrolytically precipitated on the surfaces of electrodes on the negative pole side, thereby purifying the water to be treated, the electrodes comprise titanium and have an oxide film preformed by heating having a thickness of 5 nm to 130 nm thereon, and electric current is flown between the electrodes in an amount large enough to apply a voltage capable of dielectrically breaking down the oxide film on the surfaces of electrodes on the positive pole side.