A graft copolymer satisfying (a) to (e): (a) a main chain derived from one or more monomers selected from (I) an acrylic acid, (II) a methacrylic acid, (III) a vinyl ester, (IV) a styrene and (V) a vinyl group-containing silane compound, at least one kind of the monomer having a crosslinkable functional group, (b) a side chain derived from a reactive homopolymer α-olefin having a meso pentad fraction (mmmm) of from 30 to 80% by mol or a copolymer of 50% by mass or more of at least one α-olefin, (c) a graft ratio of from 1 to 150% by mass, (d) a weight average molecular weight of from 500 to 400,000, and (e) a molecular weight distribution (Mw/Mn) of from 1.5 to 4.