PURPOSE: To keep a uniform degree of maturation at all times by detecting a generating amount of heat of an epoxy resin molding material being matured every constant time interval by a differential calory analyzer, and finishing the maturation when a level of the detected generating amount of heat reaches a previously detected level.

CONSTITUTION: A generating amount of heat of an epoxy resin material is changed while the material is matured, that is, it is high at an initial stage, immediately lowered at a reaction stage and becomes constant at a stable stage. The material in the stable stage shows a high melt viscosity and hardly generates fins when molded. A maturation degree of the resin can be detected by measuring the generating amount of heat. A differential scanning calorie analyzer is a calorimeter measuring a differential calory. A measuring time interval for the generating amount of heat is, for instance, every 1-4hours. A level is obtained by detecting an average value, e.g. from 5-10 samples. The maturation is stopped when the level of the generating amount of heat obtained as above becomes the same as a previously detected level, so that a uniform maturation degree can be maintained at all times.