Provided is an efficient treating method for coating material waste water which meets a change in a concentration of contaminants contained in waste water and a change in a treating amount by converting scarcely decomposable substances contained in the coating material waste water to easily decomposable substances. The coating material waste water is irradiated with a micro wave to decompose organic substances contained in the above waste water. Further, the coating material waste water before irradiated with a micro wave may be subjected, if necessary, to coagulation treatment by a flocculant or electrolytic treatment. After irradiating with a micro wave to decompose scarcely decomposable substances contained in the waste water to easily decomposable substances, the resulting treated water is preferably fed to a biological reaction bath and subjected to biological treatment under aerobic or anaerobic atmosphere.