A rubber-metal adhesion promoter characterized by including: a metal salt (1) of a carboxylic acid which is a metal salt of an aliphatic carboxylic acid having 2 to 25 carbon atoms and in which the metal is bismuth, copper, antimony, silver or niobium; or a compound (2) represented by the following general formula (A): [wherein Z represents a structure selected from the following formulae (z-1) to (z-4); M represents bismuth, copper, antimony, silver or niobium; (RCOO) represents a residue of an aliphatic carboxylic acid having 2 to 25 carbon atoms; and x represents the valence of M minus 1].