PURPOSE: To detect the degree of the deterioration of an actually operating apparatus by individually detecting the degrees of the deteriorations of two characteristics having the different activating energies in thermal deterioration, thereafter displaying a temperature-time curves, and obtaining the operating temperature of the actually operating apparatus based on the two curves.

CONSTITUTION: A polyethylene terephthalate (PET) film which is used as an electric insulating material is used as a sample. The change in crystal state due to deterioration is detected by the change in curve of heat-quantity measurement (DSC) for scanning indicated differences as a DSC method. The change in molecule structure mainly due to deterioration and the change in assembled state of molecules are detected by an M-tanδ method with the change in temperature distribution characteristics of dynamic tanδ(M-tanδ). Thus the degrees of the deteriorations are individually detected by using the two methods. The deterioration degrees of the two characteristics are plotted as temperature-time curves based on the Arrhenius equation. The operating temperature of an actually operating apparatus is obtained based on the intersection of the two curves. Thus the deterioration degree of the actually operating apparatus can be detected.