The polyester-based resin composition of the present invention comprises a melt blend (C) consisting of a polyamide resin (A) and a polyester resin (B). The polyester-based resin composition satisfies the following formulas 1 and 2: P×C/100≦25  (1) Y/X×100≧90  (2) wherein P is a concentration, ppm, of a phosphorus compound in the polyamide resin (A) in terms of phosphorus atom; C is a content, % by mass, of the polyamide resin (A) in the melt blend (C); X is a lightness of a 2-mm thick plate which is molded only from the polyester resin (B); and Y is a lightness of a 2-mm thick plate which is molded from the melt blend (C). By regulating the phosphorus concentration and the polyamide resin (A) content so as to meet the limitation of formula 1, the polyester-based resin composition and a shaped article or packaging container made thereof are effectively prevented from the darkening due to the deposition of antimony metal despite the blending of a polyamide resin, thereby attaining a high clearness as defined by formula 2.