PURPOSE: To obtain the title product free from spacers, by casting a thermosetting resin into a mold while supporting a transformer coil, contained in the mold, with support pins slidably passing through the mold wall, and by pulling out the pins as the coil is fixed in the gelatinized resin.

CONSTITUTION: The coil 12 is wound around the cylindrical frame 11 attached to the core 15b, supported and positioned by the support pins 13 slidably passing through the wall of the mold 15a. The core 15b is heated by electricity conduction of the heater 14 embedded therein to 20W30°C higher than the mold 15a, and a thermosetting resin, e.g. an epoxy resin, is cast from the inlet 17. The resin around the core cures rapidly, and the pins 13 are pulled out near the mold wall at the same time when the coil 12 is fixed. The resin is cast into the mold fully, and the mold and core are removed to take out the product.