Methods may include methods of producing polyester resin composition and methods of producing a polyester resin formed article that make it possible to improve thermal resistance, and to provide the polyester resin composition and the polyester resin formed article. Methods of producing a polyester resin composition may include: a step (I-a) of obtaining a polylactic acid composition (X) containing a polylactic acid (A), pentaerythritol (C), and a silicate (D); and a step (II-a) of mixing the polylactic acid composition (X) with a poly(3-hydroxyalkanoate) (B).