NOVELTY - A polyester resin composition (C1) comprises a polyester resin; a titanium compound derived from a catalyst; and a phosphorus compound. The mass content ratio of titanium element to phosphorus element in the polyester resin composition is greater than 0.10 to less than 0.20, and the increase in intrinsic viscosity of the resin composition per hour under conditions of 225 degrees C and 50 Pa is greater than 0.0090 to less than or equal to 0.0185 dL/g/hour.    USE - As polyester resin composition useful for making polyester film for solar cell; for solar cell power generation module (claimed); for photovoltaic power generation.    ADVANTAGE - The polyester resin composition exhibits excellent hydrolysis resistance, so that degradation in long time use is suppressed even with a relatively thick film thickness of 250-500 mu m. The composition has a common logarithm of R (Log R) (where R represents a volume resistivity ( Omega .m)) of composition) of less than or equal to 6.9, and when Log R is less than or equal to 6.9, electrostatic voltage application is easily achieved in a process of forming a film from the polyester resin, thus unevenness in film thickness can be reduced, and the resulting film exhibits high electrical strength when it is used as a solar cell protective film. The polyester film exhibits excellent weather resistance. When the polyester film is stored under an atmospheric condition, the storage time (retention half-life of elongation at break) until the elongation at break after storage in an atmosphere of 85 degrees C and 85% RH (relative humidity) becomes 50% of the elongation at break before storage is preferably greater than or equal to 4000 hours. The film shows an intrinsic viscosity (IV) of 0.6-0.9, which allows attaining an adequate melt viscosity during film production process, suppresses thermal decomposition of polyester caused by shearing heat generation; and suppresses acid value (AV value) low. The solar cell power generation module exhibits stable power generation performance over a long period of time.    DETAILED DESCRIPTION - INDEPENDENT CLAIMS are included for the following:    (1) producing a polyester resin composition (C1) involving i) reacting a dicarboxylic acid component and a diol component through an esterification reaction to obtain an esterification reaction product; ii) performing a transesterification reaction of the esterification reaction product to obtain a condensation polymerization product; and iii) adding a phosphorus compound to a reaction liquid, before the esterification reaction is terminated but after a titanium compound is added to the reaction liquid, where the composition satisfies the following formula i.e. mass content ratio of titanium element to phosphorus element (Ti/P) is greater than 0.10 to less than 0.20; and increase in intrinsic viscosity ( Delta IV) of the composition per hour under conditions of 225 degrees C and 50 Pa is greater than 0.0090 to less than or equal to 0.0185 dL/g/hour; and    (2) a polyester film having a thickness of 250-500 mu m comprising the polyester resin composition (C1).