PURPOSE: To enable wire to be coiled smaller as a whole and also to reduce volume of the coiling operatinon by providing an insulation layer of prepregnated sheet at least on the side of a low voltage coil, which is not adjacent to a high voltage coil, and molding these two coils as an integrated unit.

CONSTITUTION: A 130mm wide prepregnated sheet 3, made by coating epoxy resin onto a glass cloth, is wound on a 150mm steel-made cylindrical inner mold in five layers to make it 1.5mm thick, and a 1.5mm thick and 100mm wide aluminum sheet and an aromatic polyamide paper are also wound on thus prepared inner mold in four layers to make a low voltage coil 1. A columnar spacing piece of 10mm×120mm, consisting of epoxy resin containing 40vol% of silica, is placed on the coil, and a 4mm thick aluminum sheet and an armatic polyamide paper are wound in sixty layers to prepare a high voltage coil 2. A steel outer mold and a side plate are assembled and heated at 150°C for there hours for curing of the prepregnated sheet and drying of a coil conductor section. And then, thus prepared coil is impregnated with epoxy mold resin 4 in vacuum and heated for 15 hours each at respective temperatures of 120°C and 150°C for allowing it to be cured and then, as a result, it is possible to obtain a coil smaller in diameter than the conventional one by approximately 15mm.