An object of the present invention is to provide a novel method for diagnosing film degradation which can identify the degraded state of a film based on a resin more efficiently and reliably than ever. In order to attain this object, a method for diagnosing the degradation of a film based on a resin is adopted, the method comprising using the following analysis method A and/or analysis method B, which is a non-destructive analysis method: analysis method A: confirming the presence or absence of abnormality in the film by visual observation and olfactometry, and analysis method B: confirming the presence or absence of an acid anhydride and a sign of hydrolysis reaction as to the film by Fourier transform infrared spectroscopy analysis.