The present invention provides a flame retardant polyester resin composition that is free from halogen and can have a high level of initial flame retardancy and maintain flammability even after a long-term heat aging test. By allowing an organophosphorous flame retardant (B) represented by the general formula (1) below: (where n=2 to 20) and a nitrogen compound (C) to be contained at a specific ratio with respect to a thermoplastic polyester resin (A), it is possible to obtain a flame retardant polyester resin composition that can have a high level of initial flame retardancy and maintain flammability even after a long-term heat aging test.