A copolymer having silyl groups with at least one reactive functional group bonded thereto. The copolymer comprises a monomer (A) shown by the following Formula (I): ##STR1## wherein R1 is hydrogen atom or methyl; R2 is alkylene group having 1-6 carbon atoms; and R3, R4 and R5 each is a reactive functional group which can cross-link molecules of the copolymer by hydrolyzing. Further, the copolymer preferably comprises, as a constituent monomer, an alkyl (meth)acrylate and a siloxane-containing (meth)acrylate. A coating-forming method comprises hydrolyzing the composition on a material to be treated to cross-link molecules of the copolymer when on the material. A coating of the cross-linked copolymer has resistance to washing. This coating can modify the nature of hair, improve make-up retention, and provide skin-protecting. It can impart water-repellency, resistance to fouling, suitability as a sizing and crease resistance to fibers. Further, it can improve a skin-cleaning effect, applicability, strippability and coating-strength of a peeling-off type cosmetic pack.