To provide a polyester resin composition excellent in flexibility and also gas barrier properties and heat resistance, and to provide a method for producing the same composition.

This polyester resin composition consists of (B) 40-90 pts.wt. of a crystalline polyester and (C) 60-10 pts.wt. of an amorphous polyester containing a structural unit derived from a compound selected from the group consisting of phthalic acid, isophthalic acid, succinic acid, glutaric acid, adipic acid, glucolic acid, diglucolic acid, lactic acid, ethylene glycol, trimethylene glycol, 1,4-butanediol and 1,6-hexanediol in an amount of ≥85 mol% based on 100 mol% of all structural units, having at least three kinds of structural units each contained in an amount of ≥10 mol% and having a Tg of ≤20°C when measured by DSC. The above composition is produced by melting and kneading the ingredients B and C.