A treatment agent for inorganic siliconaceous fillers has as its main ingredient an organosilicon compound of general fomrula (I): ##STR1## in which X represents a functional group selected from among H.sub.2 NCH.sub.2 CH.sub.2 --, H.sub.2 NCH.sub.2 CH.sub.2 CH.sub.2 --, ##STR2## Y.sup.1 is CH.sub.2 .dbd.CHCH.sub.2 -- or --CH.sub.2 CH.sub.2 CH.sub.2 SiR.sub.n.sup.1 Z.sub.3-n.sup.1, R.sup.1 is a substituted or nonsubstituted monovalent hydrocarbon group, Z.sup.1 is an alkoxy group having 1-6 carbon atoms and n is an integer from 0-2, or of general formula (II): ##STR3## in which Y.sup.2 is ##STR4## --CH.sub.2 CH.sub.2 CH.sub.2 SiR.sub.m.sup.2 Z.sub.3-m.sup.2 or a substitutent or nonsubstituted monovalent hydrocarbon group, Z.sup.2 is an alkoxy group having 1-6 carbon atoms and m is an integer from 0-2.