PURPOSE: To eliminate paper to be inserted between layers, assure interlayer insulation and simplify insulation treatment process by simultaneously winding an insulating thread between layers of high voltage winding wound around a bobbin.

CONSTITUTION: A twisted thread 28 of nylon, Tetoron or cotton is wound simultaneously with winding of a high voltage winding 26 to the external circumference of a bobbin 22. The thread 28 is located between windings 26 and the windings are separated by the thread 28 between layers and between lines. According to such a structure, the air is exhausted through the gap formed by the thread 26 at the time of fixing the position of winding 26 with the insulating resin, generation of voids can be prevented and cost of insulating process can be reduced remarkably.