A method for producing a polyester resin composition includes conducting polyconensation via esterification or transesterification, wherein an alkali metal phosphate in an amount of 1.3 mol/ton to 3.0 mol/ton and phosphoric acid in an amount of 0.4 to 1.5 times (by mole) that of the alkali metal phosphate are added at a stage between the point of time when the esterication or transesterification has been substantially completed and the point of time when the intrinsic viscosity reaches 0.4. A polyester resin composition obtained by the process exhibits excellent long-term hydrolysis resistance and excellent mechanical characteristics.