PURPOSE: To correctly decide the preservative stability of a produced prepreg and to surely grasp the usable time of the product by collecting a part of the prepreg as a test piece and conducting a differential scanning calorimetry(DSC) on the test piece.

CONSTITUTION: Carbon fibers are arranged and held in one direction between papers coated with a resin. An epoxy resin is impregnated between the carbon fibers to produce a prepreg. According to the DSC, a part of the prepreg is collected as a test piece, and a DSC curve is prepared and an on-set value (To) and a glass transition temperature (Tg) are obtained. On the other hand, after the prepreg is stored in a chamber at a constant temperature, the To, Tg and tackiness are obtained similarly. In this manner, the To of the epoxy resin of the prepreg is compared with a predetermined reference value to decide the preservative stability of the prepreg.