Herein is disclosed a flame-retarded thermoplastic resin composition which comprises an original flame-retarded thermoplastic resin composition comprising certain amounts of a thermoplastic resin and of a brominated flame-retardant, and, in combination therewith, a certain amount of a dolomite compound, or in addition thereto, a certain amount of at least one compound selected from a hydrotalcite, a zinc-substituted hydrotalcite compound, or a zeolite compound, which flame-retarded thermoplastic resin composition of the present invention are remarkably improved in heat stabilizing effects as compared with the original flame-retarded thermoplastic resin composition.