To solve such a problem that the conventional hydrophilicizing treatment method for powdery fire-extinguishing agent wastes requires a large amount of initial expense and running cost.

A method comprises an alcohol spray step of spraying an aqueous solution of alcohol to powdery fire-extinguishing agent wastes and an agitation step of agitating the aqueous solution of alcohol sprayed in the alcohol spray step and the powdery fire-extinguishing agent wastes. The silicon coat on the surface of the powdery fire-extinguishing agent wastes is removed in the agitation step. This process enables the powdery fire-extinguishing agent wastes abundant in nitrogen and phosphorus among 3 major elements of a fertilizer; nitrogen, phosphorus and potassium to be used as a fertilizer and leads to an effective use of the powdery fire-extinguishing agent wastes roughly occurring as much as 10 thousand tons per year.