An object of the present invention is to substantially improve the compatibility between a radical copolymerizable unsaturated resin and an addition polymerized polymer (thermoplastic resin) which is added for the purpose of low profile and improvement of physical properties. The present invention provides a compatibilizing agent for compatibilizing a radical copolymerizable unsaturated resin with an addition polymerized polymer, characterized in that said compatibilizing agent is a graft copolymer (A) which contains a styrene monomer as a principal component, and has a principal chain (A1) consisting of a copolymer with a (meth)acrylate monomer and a side chain (A2) selected from a ring-opening polymerized polyether side chain consisting of a polyoxyalkylene ether, a polyester side chain, and a polycarbonate side chain, the side chain (A2) being bonded to the principal chain.