A polyester resin composition containing a polyester resin, and a fine cellulose fiber composite having an average fiber diameter of from 0.1 to 200 nm, wherein the fine cellulose fiber composite contains fine cellulose fibers and hydrocarbon groups being connected thereto via an amide bond. The polyester resin composition of the present invention can be worked into a molded article by thermoforming or injection-molding, and is suitably used for various industrial applications, such as daily sundries, household electric appliance parts, packaging materials for household electric appliance parts, and automobile parts. In addition, the fine cellulose fiber composite has high dispersibility against various resins and can exhibit thickening effects, so that it is suitable as various thickeners or the like.