There is provided by the present invention a polyester resin composition containing a copolyester having hydroxy carboxylic acid units as constituent units or a polyoxycarboxylic acid, wherein hydroxy carboxylic acid units of 5 or less carbon atoms are contained in amounts of 2 to 75% by mol based on 100% by mol of all the constituent units contained in the composition, and a molar ratio S.sub.AA of hydroxy carboxylic acid units both of whose neighboring units are hydroxy carboxylic acid units to all the hydroxy carboxylic acid units contained and a molar ratio S.sub.BB of hydroxy carboxylic acid units neither of whose neighboring units is a hydroxy carboxylic acid unit to all the hydroxy carboxylic acid units contained satisfy the following formula: 0.03