In an insulation monitoring or the like of an electric power apparatus, an antenna for measuring partial discharge of an electric apparatus is usually mounted in an inner side of the apparatus and performs transmission/reception of a signal by means of a signal processing unit and a coaxial cable placed in an outer side of the apparatus, and in addition, the electric apparatus is frequently operated being filled with air, a hydrogen gas, an SF6 gas and the like in the apparatus and being in a sealed state of its frame, so that, conventionally, it has not been possible to conveniently verify, during operations, whether the antenna and a coaxial cable mounted in an inner side of the apparatus are accurately wire-connected or not. A penetration terminal for connecting the coaxial cables inside and outside of the apparatus thereto is attached on the electric apparatus' frame, and the measurement of a combined capacitance of the antenna and the coaxial cable in the inner side of the apparatus is made possible from the outer side of the apparatus, so that it is possible to perform at any time the determination whether a wiring connection of the antenna mounted in the inner side of the electric apparatus is appropriate.