A primer for vulcanization bonding, which comprises a copolymerization copolymer of amino group-containing alkoxysilane and vinyl group-containing alkoxysilane, and an organometallic compound, can give a metal-rubber composite having a water-resistant adhesiveness equivalent or superior to that of coating type chromate processing agent, without application of harmful coating type chromate processing to a metal, and thus can be effectively used in surface treatment of metals such as stainless steel, etc. for the production of seal products of rubber-bonded metal (e.g. oil seals, packings and gaskets), valves, vibration-controlled or vibration damped products (e.g. vibration-controlled steel sheets and vibration-damped rubber), industrial rubber products, etc.