Abstract: A process for producing a polyester of uniform quality, in which a slurry of powdery aromatic dicarboxylic acid in a liquid diol is prepared and the resulting slurry is processed through an esterification step and a polycondensation step in the presence of a catalyst, is provided by performing the reactions in the esterification and the polycondensation in a stabilized condition, which process comprisesrecycling a liquid containing the unreacted diol and the catalyst separated from the reaction mixture in the polycondensation step back to the esterification step as recycling liquor,detecting the concentrations of diol, diol condensation product, catalyst and water in the recycling liquor in the course of being recycled back to the esterification step by a near infrared spectrophotometer of a noise level of at the highest 50.times.10.sup.-6 Abs with a wave length repeatability of 0.