PURPOSE: To obtain the titled smooth polyester film containing a large amount of uniformly precipitated fine particles, by adding a Ca compound and a pentavalent phosphorus compound to the reaction system in a specific state, adding a specific organo-phosphorus compound to the system, and starting the polycondensation reaction.

CONSTITUTION: The objective polyester is prepared by the polycondensation of (A) an acid component composed mainly of a lower alkyl ester of terephthalic acid and (B) ethylene glycol, wherein the polycondensation system contains (C) a calcium compound (e.g. calcium acetate, etc.) and (D) a pentavalent phosphorus compound (preferably trimethyl phosphate, etc.) added to the system in molten state prior to the polycondensation reaction, and (E) a trialkyl phosphite (e.g. trimethyl phosphite) is added to the polycondensation system. The amount of the component (E) is adjusted to give a polyester having a specific resistance of 5×108Ω-cm in molten state. The addition of the component (C) and the component (D) is preferably carried out after the esterification reaction of the component (A) and the component (B).