The invention provides a novel means for solving the problem of poor adhesion between the surface of a substrate of a metal or a plastic resin, e.g. polycarbonate resin, and a cured coating film of an organopolysiloxane formed thereon to impart good surface properties, e.g. hardness and weathering resistance, to the substrate surface by use of an adhesion improver as a primer which comprises a copolymer of an organosilicon compound having a hydrolyzable group and an ethylenic unsaturation in the molecule, glycidyl methacrylate and an organic monomer, e.g. methyl methacrylate and vinyl acetate, a reaction product of an aminoalkyl-containing organosilane compound and an acid anhydride and, optionally, an ultra-violet light absorber in a limited proportion.