An object is to obtain a flame-retardant polyester-based resin composition that uses no halogen-based flame retardant, has high flame retardance, and is excellent in strength retention after long-term heat resistance test under high temperature. The flame-retardant polyester-based resin composition according to the present invention contains 100 parts by weight of a thermoplastic polyester-based resin (A), 5 to 80 parts by weight of an organophosphorus-based flame retardant (B) represented by the following general formula 1, and 1 to 20 parts by weight of at least one amorphous thermoplastic resin (C) selected from the group consisting of a polyetherimide resin, a polysulfone-based resin, and a polyarylate resin. (wherein n is an integer of 2 to 40)