Modified polyester resin production method

A modified polyester resin production method, comprising:

• a step of allowing a polyhydroxyalkanoic acid to react with a reactive raw material satisfying at least any one of Condition (A-1) and Condition (A-2) described below, and Condition (B) described below, in the absence of a solvent and at a reaction temperature lower than a decomposition temperature of the polyhydroxyalkanoic acid,
• wherein a decomposition temperature of a polyhydroxycarboxylic acid is a decomposition temperature obtained by (I) and (II) described below,
• Condition (A-1):
• the reactive raw material contains a diol and a dicarboxylic acid component,
• Condition (A-2):
• the reactive raw material contains a polyester resin other than the polyhydroxyalkanoic acid,
• Condition (B):
• a ratio of an aromatic monomer configuring the reactive raw material in the reactive raw material is 0% by mole or more and less than 50% by mole,
• (I) an average degree of polymerization of the polyhydroxycarboxylic acid when performing a thermal treatment at a certain temperature is measured by gel permeation chromatography (GPC), and from time dependency of a reciprocal number thereof, a thermal decomposition constant at each temperature is calculated,
• (II) a temperature at which the thermal decomposition constant is 1×10−5 min−1 or more is the decomposition temperature of the polyhydroxycarboxylic acid.