An adhesion-promoting agent represented by the following average formula: R1aSiO(4-a/2 (where R1 is a group selected from an optionally substituted alkyl group with 1 to 10 carbon atoms, an alkenyl group with 2 to 20 carbon atoms, an aryl group with 6 to 20 carbon atoms, an alkoxy group with 1 to 10 carbon atoms, or an epoxy-containing organic group; however, in one molecule, the content of the alkenyl groups should constitute at least 5 mole % of all groups designated by R1; the content of the aryl groups should constitute at least 5 mole % of all groups designated by R1; the content of the alkoxy groups should constitute at least 5 mole % of all groups designated by R1; the content of the epoxy-containing organic groups should constitute at least 5 mole % of all groups designated by R1; and “a” is a number that satisfies the following condition: 1.0≦a