According to a coil deterioration diagnostic method of the present invention, a conductor to be cooled directly by a medium is covered with an insulation layer, a coil is constituted of internal electrodes to be wrapped in an insulation layer, an AC voltage is applied to the conductor of the coil, a potential of the internal electrode is measured by a potential measuring probe facing the coil, and if a measured potential is higher than the potential of a sound coil, it is discriminated that the medium leak out from the conductor to the insulation layer and an insulation thereof is deteriorated. According to above-mentioned structure, an improved coil deterioration diagnostic method and an improved coil deterioration diagnostic apparatus applied to this diagnostic method can be provided, capable of detecting, easily, accurately and securely through an easy procedure, an insulation deterioration on a coil generated by leakage of water from a conductor to an insulation layer upon operation with the conductor cooled directly by water.
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