Adhesion Accelerator

A method of improving the bonding of vulcanized rubber to metal, in which the improvement is characterized in an adhesion accelerator comprising at least one nickel organic compound selected from the group consisting of Ni-containing soaps having carboxylic acid moieties each having 6 to 30 carbon atoms in its main chain, or a mixture thereof, an organic boron complex containing Ni bonded to a carboxylic acid moiety having 6 to 30 carbon atoms in its main chain, and Ni-acetyl acetonate, and at least one zinc organic compound selected from the group consisting of Zn-containing soaps having carboxylic acid moieties each having 6 to 30 carbon atoms in its main chain, or a mixture thereof, an organic boron complex containing Zn bonded to a carboxylic acid moiety having 6 to 30 carbon atoms in its main chain, and Zn-acetyl acetonate, blended together to have a Ni/Zn weight ratio of from 0.01 to 20, being added in an amount of 0.1 to 10 parts by weight, based on 100 parts by weight of the rubber component, to a rubber component and 3 to 8 parts by weight of sulfur, based on 100 parts by weight of the rubber component, prior to the vulcanization thereof.