TECHNOLOGY FOCUS - POLYMERS - Preferred Process: In the process, a phosphorus compound is added to a reactant, before termination of the esterification reaction and after the addition of the titanium compound in the step (a). The solid phase polymerization is performed under pressure of 1-500 Pa or under a nitrogen atmosphere at 200-230 degrees C. Preferred Components: The polyester resin composition further comprises a phosphorus compound. The titanium compound comprises an organic chelate titanium complex having an organic acid as a ligand. The phosphorus compound is of formula ((R1O)3P=O) (I). The content of the titanium compound is greater than 1 ppm to less than or equal to 30 ppm, the content of phosphorus compound is greater than 50 ppm to less than or equal to 90 ppm, and the content of titanium and phosphorus compounds are 0.10-0.20, in terms of titanium element or phosphorus element. The amount of a carboxy end group is not greater than 25 molar equivalent/ton and intrinsic viscosity is 0.60-0.90. The polyester resin composition further comprises an amount of greater than or equal to 50 ppm in terms of the metal element equivalent (by mass) of a compound comprising at least one metal element including alkali metals, alkaline earth metals, the iron group, manganese, tin, lead or zinc. The polycondensate that is subjected to the solid phase polymerization has a specific surface area of 500-2000 m2/m3. In the polyester film, a preservation time in which fracture elongation of the polyester film after preservation is 50% with respect to that before preservation is not less than 2000 hours when the polyester film is preserved in an atmosphere of 85 degrees C and relative humidity of 85%.R1 = 1-3C alkyl. EXAMPLE - High purity terephthalic acid (4.7 tons) and ethylene glycol (1.8 tons) were mixed over 90 minutes to form slurry. The slurry was continuously supplied at a flow rate of 3800 kg/hour to a first esterification reactor. Then, an ethylene glycol solution of a citric acid chelate titanium complex having citric acid coordinated to titanium metal was supplied continuously. The reaction was carried out while stirring under at 250 degrees C in a reactor and at an average retention time of 4.3 hours. The citric acid chelate titanium complex was continuously added such that an addition amount of the titanium complex was 9 ppm in terms of titanium element. The resulting reaction mixture was transferred to a second esterification reactor tank, and reacted while stirring under at 250 degrees C in a reactor and at an average retention time of 1.2 hours. The inside of the second esterification reactor tank was parted into three zones. From a second zone, an ethylene glycol solution of magnesium acetate was continuously supplied such that the addition amount of magnesium acetate was 75 ppm in terms of magnesium element. Sequentially, from a third zone, an ethylene glycol solution of trimethyl phosphate (adding a trimethyl phosphate liquid of 25 degrees C to an ethylene glycol liquid of 25 degrees C, and stirring them at 25 degrees C for 2 hours) was continuously supplied such that the addition amount of trimethyl phosphate was 65 ppm in terms of phosphorus element, In this manner, an esterification reaction product was obtained, where the element content ratio of titanium and phosphorus was 0.14. The esterification reaction product was supplied continuously to a first condensation polymerization reactor tank. Condensation polymerization (transesterification reaction) was carried out while stirring under conditions of temperature of 270 degrees C, a reactor inside pressure of 20 Torr in a reactor, and average retention time of 1.8 hours. Then, the reaction product was transferred from the first condensation polymerization reactor tank to a second condensation polymerization reactor tank. In this reactor tank, reaction (transesterification reaction) was carried out while stirring at 276 degrees C and at pressure of 5 Torr in a reactor, and retention time of 1.2 hours. Sequentially, the reaction product was transferred from the second condensation polymerization reactor tank to a third condensation polymerization reactor tank. In this reactor tank, reaction (transesterification reaction) was carried out at 278 degrees C, pressure of 1.5 Torr in a reactor and retention time of 1.5 hours, to obtain a polycondensation product (polyethylene terephthalate (PET)). Then, the resulting polycondensation product (PET) was extruded into cold water in a strand form and immediately cut, to obtain polyester resin composition pellets.