PURPOSE: To obtain the titled polymer, having excellent mechanical strength, heat resistance, etc., and suitable as fibers, films, machine parts, etc., by copolymerizing polyethylene terephthalate with an aromatic carboxylic acid and specific compound.

CONSTITUTION: (A) Polyethylene terephthalate having preferably 0.1W1.0dl/g reduced viscosity is mixed with (B) m-acetoxybenzoic acid and (C) a compound capable of providing structural units expressed by formula I (R1 and R2 are H, 1W5C alkyl, etc.; X is O, S etc.) and (D) 1,4-bis[(4-carboxy)phthalimido]-2- chlorobenzene obtained by reacting trimellitic acid with a 1,4-diaminobenzene derivative, reacted in an inert gas, e.g. argon, at 250W350°C and then polymerized under reduced pressure to afford the aimed polymer having repeating units expressed by formula IWIV (R3 is H, etc.) and ≥0.2dl/g intrinsic viscosity.