To provide a primer composition effective for favorably bonding a cured product of curable composition crosslinked by hyrosilylation reaction to various substrates and a bonding method.

This primer composition comprises (A) a saturated hydrocarbon- based polymer having at least one alkenyl group in one molecule, (B) a silane coupling agent, (C) a polyvalent alkoxysilane and/or a condensation product thereof and (D) an organoaluminum compound and/or an organotitanium compound or this primer composition comprises the components (B), (C) and (D) and further (E) a vinyl copolymer having a backbone structure substantially comprised of a vinyl copolymer chain and containing a silicon-containing group having a hydroxyl or hydrolyzable group bound to a silicon atom and capable of crosslinking by siloxane bonding terminally of its backbone chain and/or in its side chain. This method for bonding the cured product to a substrate comprises coating a primer composition comprising the component(B) or the components (C) and (D) on a substrate and applying a curable composition comprising a saturated hydrocarbon-based polymer having at least one alkenyl group in the molecule onto the primed substrate.