Abstract: A method for diagnosing an insulation deterioration of a power cable comprising a step of measuring a DC current component in the AC voltage applied to said power cable, by using a filter circuit for cutting off an AC current and a DC current measuring apparatus for measuring small magnitude of DC current. According to the present invention, said filter circuit and said DC current measuring apparatus are joined in parallel each other and connected between a power source and one end portion of said power cable to be diagnosed. The DC current measuring apparatus can detect the polarity, magnitude, and time lapse characteristics of said DC current component for analyzing and estimating the insulation deterioration.