PURPOSE: To make it possible to easily judge the dielectric breakdown strength between the layers of a coil, by preliminarily providing a minute electrode apparatus, which has a pair of electrodes communicated with the air layers between coil inner layers, in a primary coil and calculating the dielectric breakdown strength between the electrodes.

CONSTITUTION: A minute electrode apparatus 11 is preliminarily embedded in a primary coil 4 and this coil 4 is cast along with an iron core 1 and a secondary coil 2 by an insulating material. Voltage is applied between the electrodes of this apparatus 11 through lead wires 14 to measure the dielectric breakdown strength between the electrodes 13 within a space part 17. Because the space part 17 and an air layer are set to the same vacuum degree by a communication hole 16, the dielectric breakdown strength of the air layer can be known from that between electrodes 13. Further, the dielectric breakdown strength between the layers of the coil can be also calculated from said dielectric breakdown strength. By this method, the measurement of the optimum dielectric breakdown strength between the layers of the coil can be easily performed.