The present invention provides an adhesion promoter for promoting adhesiveness of a cured product of an addition curable organopolysiloxane resin composition, the adhesion promoter comprising at least one alkoxy group bonded to a silicon atom, and at least one cyano group-containing organic group bonded to a silicon atom in one molecule. There can be provided an adhesion promoter which can make a cured product of an addition curable organopolysiloxane resin composition excellent in heat resistance, workability, adhesiveness, and gas barrier property by adding it to the composition.