A graft copolymer is provided that is useful as an adhesive composition that is excellent in adhesion strength between polyolefins and between a polyolefin and another member (for example, a metal, a resin, and an inorganic material, such as glass), is excellent in flowability at a low temperature and in melting property in a low temperature range, and is excellent in heat resistance after adhesion, i.e., a graft copolymer satisfying the following items (a) to (e):the graft copolymer having:(a) a main chain that is derived from one kind or two or more kinds of monomers selected from (I) acrylic acid and a derivative thereof, (II) methacrylic acid and a derivative thereof, (III) a vinyl ester and a derivative thereof, (IV) styrene and a derivative thereof and (V) a vinyl group-containing silane compound, at least one kind of the a monomer having a crosslinkable functional group,(b) a side chain that is derived from a reactive polyolefin having a meso pentad fraction (mmmm) of from 30 to 80% by mol and having an end unsaturated group in an amount of from 0.5 to 1.0 group per one molecule, the reactive polyolefin being a homopolymer of one kind of an α-olefin having from 3 to 28 carbon atoms, a copolymer of two or more kinds of α-olefins having from 3 to 28 carbon atoms, or a copolymer of 50% by mass or more of at least one of an α-olefin having from 3 to 28 carbon atoms and 50% by mass or less of ethylene,(c) a graft ratio of from 1 to 150% by mass,(d) a weight average molecular weight of from 500 to 400,000 measured by GPC, and(e) a molecular weight distribution (Mw/Mn) of from 1.5 to 4.