NOVELTY - The polyester film comprises a polyester resin, and a trifunctional or more polyfunctional constitutive ingredient in which the total of the carboxyl acid groups and the hydroxyl groups in one molecule is 3 in an amount of 0.005-2.5 mol % of the polyester film. The polyester film has a specific heat change ( Delta Cp) of 0.06-0.1 J/g at 85-135 degrees C, a thermal shrinkage of 0.05-0.5%, and a haze of 0.1-40%, and is produced by forming a polyester film through melt extrusion of a polyester resin, stretching the polyester film, and thermally fixing the stretched polyester film.    USE - The polyester film is useful in a backsheet for solar cells and a solar cell module (all claimed).    ADVANTAGE - The polyester film can be effectively and simply manufactured with excellent weather resistance, good planar surface state, less thermal shrinkage and long life span.    DETAILED DESCRIPTION - The polyester film comprises a polyester resin, and a trifunctional or more polyfunctional constitutive ingredient in which the total of the carboxyl acid groups and the hydroxyl groups in one molecule is 3 in an amount of 0.005-2.5 mol % of the polyester film. The polyester film has a specific heat change ( Delta Cp) of 0.06-0.1 J/g at 85-135 degrees C, a thermal shrinkage of 0.05-0.5%, and a haze of 0.1-40%, and is produced by forming a polyester film through melt extrusion of a polyester resin, stretching the polyester film, and thermally fixing the stretched polyester film. The polyester resin has an intrinsic viscosity (IV) of 0.73-0.9 dL/g, and a terminal carboxyl group content of 5-21 eq/ton. The heat treatment time for which the polyester film is heat-treated at 120 degrees C and at a relative humidity of 100%, and maintains 50% retention of elongation at break after the heat treatment is 75-150 hours. The film is given a local temperature fluctuation of 0.1-10 degrees C in the film width direction or a wind speed fluctuation of 0.1-10% in the film width direction. An INDEPENDENT CLAIM is included for a method for producing a polyester film.