The copolymer which is usable in the present invention denotes copolymers being composed of an aminoalkyl acrylate and/or aminoalkyl methacrylate represented by the formula ##STR1## wherein R1 is hydrogen or methyl group; R2 is an alkylene or hydroxyalkylene group of 1 to 6 carbon atoms; R3 is hydrogen or an alkyl group of 1 to 6 carbon atoms