PURPOSE: To obtain a product having a definite high crystallinity in a short time by quickly estimating the physical properties of a reaction system necessary for control and quickly and automatically controlling the reaction condition.

CONSTITUTION: This plasticizer is produced by using a polybasic carboxylic acid and an alcohol as raw materials and subjecting the components to esterification reaction and distillation. In the above process, (a) the near infrared absorption spectrum of the reaction liquid is measured at a prescribed time interval with a near infrared spectroscopic analysis equipment 10 furnished with a multivariate analysis equipment and the acid value and the residual alcohol amount of the reaction liquid are calculated from the measured data, (b) control information is produced from the obtained calculation values using an arithmetic unit and (c) the pressure of the reaction tank, the amount of steam and the reaction time are controlled according to the information.