PURPOSE: To obtain a polyester fiber reduced in carboxyl end group concentration and having good heat resistance and strength characteristics by adding a specific dilactim ether compound to a high-molecular weight polyester, reacting the dilactim ether compound with the polyester and extruding the reaction product through a spinning inlet.

CONSTITUTION: One or more kinds of dilactim ether compounds represented by the formula [(x) and (y) are each an integer of 2-15; R1 and R2 are each H, a halogen, nitro, amine, a 1-10C alkyl, a 5-12C cycloalkyl, a 7-9C alkylaryl or a 6-10C aryl] (e.g. -caprolactim ether dihydrochloride) are mixed and reacted with melted high-molecular weight polyethylene terephthalate and the reaction product is spun by extruding it through a spinning inlet to provide the objective fiber reduced in carboxyl end group concentration.