Résumé dans la langue de publication, traduction non disponible The present invention relates to a viscosity index improver to be added to lubricating oils and the like. This viscosity index improver contains a graft copolymer having a structural unit formed by an oil-soluble polymer (a) containing an olefin polymer and a structural unit formed by a copolymer (b) of a peroxy bond-containing polymer with a (meth)acrylate and the like or a copolymer (c) of a peroxy bond-containing monomer with 2-hydroxyethyl (meth)acrylate and the like. The viscosity index improver has excellent viscosity index improving property, excellent thickening effect, excellent low-temperature fluidity and excellent shear stability and is also free from the fear of separation in lubricating oils.