A very effective method is proposed for the shrink-proof treatment of a fabric material of keratinous fibers, e.g., wool, in which the fabric material is first soaked with an aqueous solution of a persulfate compound followed by drying to effect surface oxidation of the fibers and then finished with a curable organopolysiloxane composition. Preferably, the curable organopolysiloxane composition comprises, as a dispersion in an aqueous medium, (A) an organopolysiloxane having hydroxyl or alkoxyl groups, (B) silica or an organopolysilsesquioxane in a finely divided form, (C) an alkoxy silane having an amido group and a carboxyl group in a molecule, (D) an alkoxy silane having an amino group or an epoxy group in a molecule, and (E) a curing catalyst, each in a specified proportion.