The object of the invention is to provide a method for cleaning circulation water, which reduces the cost of operation and maintenance as much as possible, without a cumbersome cleaning operation such as by detaching electrode plates from an electrolysis cleaning tank and removing scale from inside the tank, and to provide a device used in this method. Accordingly, the present invention provides a method for cleaning circulation water, comprising the steps of flowing circulation water to be cleaned between electrode plates placed in a face-to-face manner between one another; and applying DC voltage between the electrode plates, allowing one or more metal ions contained in the circulation water to be precipitated onto the negative electrode plates by electrolysis, to clean the circulation water, wherein the electrode plates used are made of titanium plates, and wherein a desired amount of current flows, while an anodized oxide coating formed on the surface of the positive electrode plates is compulsively subject to dielectric breakdown by increasing the voltage applied to the anodized oxide coating. FIG. 1 is a representative drawing.