The purpose of the present invention is to provide a polyester resin with improved mechanical properties, molding performance, and, preferably, fire retardancy. This polyester resin composition contains the following: a polyester resin (A) that has a melting point or glass transition temperature of at least 250° C.; a thermoplastic resin (B) that has an olefin-derived constitutional unit and an aromatic hydrocarbon structure and has an intrinsic viscosity ([η]), as measured in decalin at 135° C., of 0.04 to 1.0 dl/g; and a copolymer (C) that has an olefin-derived constitutional unit, an α,β-unsaturated-carboxylic-ester-derived constitutional unit, and a cyclic oxy-hydrocarbon structure.