An epoxy resin curing composition comprising a novel curing agent which is obtained by the amidation with elimination of alcohol between an adduct of a carboxyl-containing acrylonitrile-butadiene copolymer with an epoxidized fatty acid ester and an amide-forming nitrogenous compound. This composition exhibits improved adhesion involving peel strength to plastics, rubbers or flexible vinyl chloride resins and is excellent in mechanical strength and resistance to water and alkali and particularly to acid.