PURPOSE: To prepare a polyester having reduced terminal carboxyl group and high heat stability, by reacting a linear saturated polyester with a bisoxazoline compound in the presence of a quaternary ammonium salt compound in a molten state.

CONSTITUTION: A substatially linear saturated polyester is reacted with 0.1W 5wt% bisoxazoline compound shown by the formula (R is bifunctional organic group; n is 0 or 1; H may be replaced with alkyl or aryl) in the presence of 0.001W0.1wt% quaternay ammonium salt compound in a molten state. The reaction between the polyester and the bisoxazoline compound is carried out when the intrinsic viscosity of the polyester is 0.50, and the reaction is done for ≥3min.