Because a huge number of ground coils are used outdoors for an extended period of time in the superconducting magnetically levitated transportation (Maglev) system, high reliability of ground coils and the cost reduction are required. The propulsion coil especially needs the high insulation stability as high-voltage equipment. Because the ground coil, which is made with the resin-molded winding, does not have a core in the structure, the ground coil must be exposed to the electromagnetic force directly. Therefore, there is a possibility that minute defects in the molded resin of the ground coil may expand, become apparent, and lead to the deterioration of insulation stability. In this paper, we examined the insulation diagnosis to assess the existence/nonexistence of the inside defect and locate the defect by setting a simulated void-defect inside the molded resin of the ground coil and detecting the electromagnetic waves effectively that are emitted from a partial discharge at the defect point.