NOVELTY - A flame-retardant polyester comprises co-polyester containing copolymer component of phosphorus compound as glycol component and dicarboxylic acid component consisting of greater than or equal to 90 mol.% aromatic dicarboxylic acid component and greater than or equal to 90 mol.% less than or equal to 4 carbon-containing aliphatic dicarboxylic acid component. The phosphorus compound contains 1500-50000 ppm in conversion of phosphorus atom with respect to polyester.    USE - A flame-retardant polyester for fire retardant polyester film used for front and back sheets of solar cell (all claimed). Can also be used for building materials, such as packaging material, membrane switch, touch panel, projector screen, label, wallpaper, and steel plate laminate, process papers, such as recording papers for printers e.g. image-receiving paper for inkjets, fuel cell, coating material, such as adhesive tape and electronic component, flat cable, layer insulation object of molded coil of a mold transformer, printed wiring board, and reflecting plate of illumination.    ADVANTAGE - The polyester and polyester film have excellent flame retardance and water resistance. The film has excellent transparency, only slight yellowishness, excellent white property, and high reflectivity.    DETAILED DESCRIPTION - A flame-retardant polyester comprises co-polyester containing copolymer component of phosphorus compound of formula (1) as glycol component and dicarboxylic acid component consisting of greater than or equal to 90 mol.% aromatic dicarboxylic acid component and greater than or equal to 90 mol.% less than or equal to 4 carbon-containing aliphatic dicarboxylic acid component. The phosphorus compound contains 1500-50000 ppm in conversion of phosphorus atom with respect to polyester.    R1, R2=1-4C alkyl, Cl, or Br;    l, m=0-4 in an averaged value;    A=ethylene, 1,2-propylene, 1,3-propylene, or 1,4-butene; and    n=1-2 with the averaged value.    An INDEPENDENT CLAIM is included for fire retardant polyester film comprising resin which has flame-retardant polyester.