Flame-retardant polyester and process for producing the same

To easily obtain a polyester having excellent mechanical properties, a satisfactory hue, and a high degree of flame retardancy. The flame-retardant polyester is a polyester comprising ethylene terephthalate units as the main structural units and a phosphorus compound copolymerized or incorporated therein or is a resin composition comprising polyesters including that polyester. The flame-retardant polyester is characterized in that a polycarboxylic acid and/or polyol having three or more functional groups capable of forming an ester bond is contained in the polyester in a total amount of 0.05-2.00 mol (per 200 mol of the sum of the dicarboxylic acid ingredient and the diol ingredient), that a specific phosphorus compound having a functional group capable of forming an ester bond is contained in the polyester in an amount of 5,000-50,000 ppm of the polyester in terms of phosphorus amount, and that the polyester pellet obtained has a b value of -5 to 20, an L value of 35 or larger, and a melt viscosity at 280 DEG C of 1,000-20,000 dPas.