This invention provides a novel polyolefin modified with a nitrogen-containing heterocyclic compound containing no harmful halogen atoms such as chlorine, and an adhesive composition between a polyolefin substrate and an inorganic substrate such as a glass substrate. A polyolefin modified with a nitrogen-containing heterocyclic compound (C) having a nitrogen-containing heterocyclic compound (B) at the side chain of an acid-modified polyolefin (A).