PURPOSE: To eliminate a difference in the initial concentration of a basic substance and the concentration thereof near the end point even at a low bath ratio, avoid remarkably deteriorating the fiber strength, improve the accuracy in weight reduction of a knitted or a woven polyester fabric and reduce the amount of waste water.

CONSTITUTION: This method of the weight reduction treatment of a knitted or woven polyester fibers is to charge a prescribed amount of a caustic soda solution into a reactional vessel housing a knitted or a woven fabric containing polyester fabric, measure its concentration, store the resultant concentration as the reference value CB in a central processing unit, calculate the theoretical required amount of the caustic soda V or V' within the tolerance of errors in the weight reduction ratio based on the weight of polyester fibers in the knitted or woven fabric and the target weight reduction ratio, store the resultant amount, then increase the temperature in the reactional vessel, start the weight reduction processing, periodically or continuously measure the concentration of the caustic soda solution during the treatment or physical values changed thereby, input the measured data to the central processing unit, output a command to inject the caustic soda solution if there is a difference between the already measured value and the reference value CB, integrate the data, reduce the temperature of the vessel when the integrated value attains the required amount V or V' and stop the weight reduction treatment.