An additive having an improved dispersibility or adhesion property to thermoplastic resins and useful for imparting an excellent flame resistance and other properties thereto without deteriorating properties that the thermoplastic resins originally possess, such as mechanical, electrical and thermal properties and molding processability, which comprises a nitrogen-containing heterocyclic compound or its salt surface-treated with 0.1 to 50% by weight of a compound having at least two functional groups such as epoxy resin, acid anhydride or the like; and a flame retardant polyester resin composition comprising (A) a thermoplastic polyester, (B) a nitrogen-containing heterocyclic compound or its salt, (C) a compound having at least two functional groups and optionally (D) a phosphorus based flame retarder, which may be prepared by adding the component (B) previously surface-treated with the component (C) or separately adding the components (B) and (C) to the polyester (A).