NOVELTY - The method comprises: subjecting a polyester raw material resin; forming an unstretched film by cooling and solidifying the melt extruded polyester resin on a cast roll; subjecting the unstretched film to biaxial stretching in a longitudinal direction and a lateral direction; heat fixing the stretched film formed by biaxial stretching; subjecting the heat fixed stretched film to a relaxation treatment in the longitudinal direction and the lateral direction of the film. The raw material resin contains a titanium compound as a polymerization catalyst.    USE - The method is useful for producing a polyester film, which is useful in a back sheet and a solar cell module (claimed).    ADVANTAGE - The method is capable of easily, effectively and economically producing polyester film with improved dimensional stability, strength, long service life, hydrolysis resistance and voltage resistance.    DETAILED DESCRIPTION - The method comprises: subjecting a polyester raw material resin; forming an unstretched film by cooling and solidifying the melt extruded polyester resin on a cast roll; subjecting the unstretched film to biaxial stretching in a longitudinal direction and a lateral direction; heat fixing the stretched film formed by biaxial stretching; subjecting the heat fixed stretched film to a relaxation treatment in the longitudinal direction and the lateral direction of the film. The raw material resin contains a titanium compound as a polymerization catalyst, and has an intrinsic viscosity of 0.71-1.00 to melt extrusion using a twin-screw extruder (100), which includes a cylinder; two screws (28) disposed inside the cylinder; and a kneading disk unit disposed in a portion of a region of 10%- to 65%-position of screw length with respect to an upstream end of the screws in a resin extrusion direction as a starting point at a maximum shear rate generated inside the twin-screw extruder of 10-2000 sec-1. The melt extrusion comprises: using a kneading disk unit having a length of 1-30% in a longitudinal direction of the screw; and performing suction through vents provided on a cylinder (10) of the twin-screw extruder. The twin-screw extruder comprises a gear pump for extrusion control that controls an extrusion output of the resin in a downstream of the cylinder in the resin extrusion direction, and a filter for foreign material removal, which removes foreign materials from the resin. The step of forming the unstretched film comprises cooling and solidification in a region in which a temperature of the polyester resin that is melt extruded from the twin-screw extruder is 140-230 degrees C at an average cooling rate in a range of 230-500 degrees C./min. The relaxation treatment is performed in the longitudinal direction of the stretched film by clamping two edges in the width direction of the stretched film using clips installed in a pair of flexurally movable clip chains to which chain links are linked in a cyclic form causing the stretched film to have a bendable structure between the clips along guide rails to cause displacement of the bending angle of the chain links, and for shortening the distance between clips in the clip run direction. An amount of terminal carboxylic acid groups in the polyester raw material resin is 8-25 eq/ton. The polyester raw material resin contains recovered waste of a polyester resin in an amount of 0-15% by mass relative to a total mass of the polyester raw material resin. An INDEPENDENT CLAIM is included for a polyester film.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a twin-screw extruder.    Cylinder (10)    Hopper (12)    Extrusion port (14)    Die (40)    Screws (28)    Twin-screw extruder. (100)