To produce a polyester excellent in transparency and color tone by a method using antimony trioxide as a polycondensation catalyst and simultaneously using a phosphorous compound.

Polyethylene terephthalate or a polyester consisting mainly of the polyethylene terephthalate is produced. Therein, the polycondensation reaction is carried out by adding antimony trioxide in an amount of 1×10-4 to 4×10-4 mole/mole of an acid component to the polyester oligomer and further adding the below-described phosphorous compound solution in an amount of ≥0.5 fold-mole that of the antimony trioxide to the polyester oligomer. The phosphorous compound solution has a water fraction of ≤1000 ppm and is obtained by dissolving phosphoric acid in a 2-10C alkylene glycol so as to give a concentration of 1-10 wt.%, and subsequently subjecting the solution to a reaction at a temperature of 150-200°C, until at least the reaction solution does not exhibit the second equivalent point on a potentiometric titration, while by-produced water is distilled out.