NOVELTY - A polyester composition comprises phosphoric acid, alkali metal phosphate with phosphorus content (P) of 1.8-5 mol/t with respect to total polyester composition, and 5 ppm or less of manganese or calcium with respect to content of divalent metal elements other than manganese and calcium. The ratio of content (P) of phosphorus with respect to metal content (M) in polyester composition is 1.1-3 mol/t.    USE - Polyester composition is used for manufacture of film for manufacturing sheet-like structure e.g. solar cell back sheet (all claimed). Uses include but are not limited to motor for car-conditioner, drive motors, capacitor material, motor vehicle material, building material, water heater motors and hybrid vehicles.    ADVANTAGE - The polyester composition has excellent high heat-and-moisture resistance.    DETAILED DESCRIPTION - A polyester composition comprises phosphoric acid, alkali metal phosphate with phosphorus content (P) of 1.8-5 mol/t with respect to total polyester composition, and 5 ppm or less of manganese or calcium with respect to content of divalent metal elements other than manganese and calcium. The ratio of content (P) of phosphorus with respect to metal content (M) in polyester composition is 1.1-3 mol/t. The metal content (M) in polyester composition satisfies a relation: M=M1/2+M2, where M1 is content of alkali metal with respect to polyester composition, and M2 is total content of manganese and calcium with respect to polyester composition. INDEPENDENT CLAIMS are included for the following:    (1) film, which has layer (P) containing polyester composition;    (2) sheet-like structure, which has film having layer (P) as core layer, layer (b1) having polyarylene sulfide as main component or aramid laminated on one surface of the layer (P), and layer (b2) essentially comprising polyarylene sulfide as main component, or aramid laminated on other surface of the layer (b2). The ratio of total thickness of layers (b1) and (b2) with respect to thickness of layer (P) is 0.1-4 mu m;    (3) solar cell back sheet, which contains polyester composition;    (4) manufacture of polyester composition, which involves transesterifying dimethyl ester of dicarboxylic acid and dialcohol in presence of manganese acetate and/or calcium acetate as catalyst, polymerizing after adding phosphoric acid alkali metal salt (0.5-3 mol/t) and phosphoric acid whose content is 0.4-1.5 times with respect to content of phosphoric acid alkali metal salt;    (5) manufacture of film, which involves biaxially-stretching the polyester composition after polymerization process, and molding in sheet form, and heating at temperature which satisfies a relation: Tm-Th-45-90 degrees C, where Tm is melting point of polyester component and Th is heat processing temperature; and    (6) manufacture of sheet-like structure.