The application possibility of an optical diagnosis was examined as simple and non-destructive technique for diagnosing the degree of thermal deterioration of molded transformers. The optical diagnosis is the technology of estimating the degree of the thermal deterioration of organic insulating resins from the reflective absorbance difference between two wavelengths containing near-infrared light, and life prediction is also possible by using chemical kinetics. First, as a result of investigating the correlation with the glass transition temperature related to the degree of the thermal deterioration of epoxy resins obtained from the dynamic viscoelastic characteristics and the reflective absorbance difference which was a diagnostic parameter, with the correlation coefficient of 0.9 or more it was theoretically judged that an optical diagnosis was applicable. Then, the diagnostic master curve was obtained from the accelerated thermal degradation examination using model samples. Next, the optical fiber cable and the probe of a portable optical diagnostic apparatus were improved using bundle fibers. As a result of evaluating six sets of operating transformers using the improved diagnostic apparatus, it has confirmed that the residual life could be estimated.