The present invention provides a method of producing a polyethylene terephthalate graft copolymerized resin, comprising subjecting the following mixture to a homogeneous reaction at a temperature of 250° C. or higher to impart a high melt viscosity to the mixture, the mixture containing (A) 100 parts by weight of a polyethylene terephthalate polyester as a main material, (B) 0.01 to 2 parts by weight of a polymeric polyfunctional epoxy compound containing 7 to 100 epoxy groups in a molecule and having a molecular weight of 1,000 to 300,000 as a coupling agent, (C) 0.01 to 1 part by weight of a metal salt of an organic acid as a coupling reaction catalyst, and (D) 0 to 100 parts by weight of an aromatic alicyclic polyester and/or an aromatic aliphatic polyester each having a carboxyl group in an aromatic nucleus as an auxiliary material.