PURPOSE: To obtain a copolymer having improved melt processability, by adding a specified compound having ester bonds and amide bonds in the production of polyester.

CONSTITUTION: A copolyester, intrinsic viscosity ≥0.4, prepared by adding a compound represented by formulaI, wherein R is a 2C or lower hydrocarbon group or H, X is an ester bond, Y is an amide bond, Ar and A'r are each an aromatic ring, or a cyclohexane ring and n is 1W3, in an amount of 0.1W1wt%, based on the total dicarboxylic acid component, in the production of polyester. As compounds represented by formulaI, there can be mentioned, for example, compounds of formulas II and III. As the polyesters used, there are preferred polyalkylene terephthalates such as polyethylene terephthalate or polybutylene terephthalate. With respect to such polyalkylene terephthalates, those in which a terephthalic acid component constitutes at least 90mol% of the total acid component are preferred from the viewpoint of heat stability, crystallinity, etc.