A modified olefin-based copolymer produced by graft-polymerizing an alkenyl aromatic hydrocarbon and/or an unsaturated carboxylic acid or derivative thereof to an olefin-based copolymer containing repeating units derived from a linear .alpha.-olefin and/or ethylene copolymer of ethylene and repeating units derived from a vinyl compound (I) or (II) described below, and an adhesive containing the product as an effective ingredient, in which an adhesiveness to an adherend such as olefin resins (e.g. polyethylene, polypropylene) is further improved, are provided.Vinyl compound (I): a vinyl compound represented by the general formula CH.sub.2.dbd.CH--R, wherein steric parameters Es and B1 of the substituent R are respectively -1.64 or less and 1.53 or more.Vinyl compound (II): a vinyl compound represented by the general formula CH.sub.2.dbd.CH--R', wherein a substituent R' is a secondary or tertiary alkyl group.