To provide a method for stably producing a polyester resin in high quality, which is subjected to depolymerization reaction or modification reaction using a polycarboxylic acid and/or polyhydric alcohol.

The method for producing a polyester resin includes, in a method for producing a copolyester in which a depolymerization reaction or modification reaction is carried out using a polycarboxylic acid and/or polyhydric alcohol after condensation polymerization reaction of polyester resin, reducing the pressure with stirring after the depolymerization reaction or modification reaction and adjusting the percentage of rise of intrinsic viscosity [η] before and after the reduction in pressure to ≤2%.