"(R4)f(R5)gSi(OR6)4???(f+g)???(III) wherein each of R4 and R5 is an alkyl group, an aryl group, a halogenated alkyl group, a halogenated aryl group, an alkenyl group or an organic group which has an epoxy group, a (meth)acryloxy group, a mercapto group or a cyano group and is to be bonded to a silicon atom by an Si-C bond, R6 is an alkyl group, an alkoxyalkyl group or an acyl group, and each of f a" . . . . .