Disclosed herein is an aliphatic polyester resin composition that simultaneously improves slow crystallization, low impact strength, and low tensile elongation that are the drawbacks of a polyhydroxyalkanoate.The aliphatic polyester resin composition comprises: a polyhydroxyalkanoate (A); a graft copolymer (B); and pentaerythritol (C), wherein the graft copolymer (B) is obtained by graft polymerization of a vinyl-based monomer onto at least one rubber selected from the group consisting of a composite rubber (b1) comprising a polyorganosiloxane component and a polyalkyl(meth)acrylate component, an acrylic rubber (b2), and a diene-based rubber (b3) comprising a diene-based monomer.