PURPOSE: To prepare a polyester having excellent particle dispersibility and suitable as a textile or film having high transparency and slippery, by adding a specific silicon oxide prepared by dry process to the polyester during polymerization.

CONSTITUTION: The objective polyester prepared from terephthalic acid or its ester-forming derivative and a glycol selected from ethylene glycol and 1,4-butanediol or its ester-forming derivative, is prepared by adding 0.05W4wt%, based on the obtained polyester, of silicon oxide having alkyl groups and blocked silanol groups on the particle surface and average primary particle diameter of ≤100mμ and obtained by dry process, to the polymerization system at an arbitrary stage before the completion of the polymerization, and then completing the polymerization.