PURPOSE: To improve workability and corona-proof effect by winding a corona- proof tape having a half-cured front surface and a cured back surface.

CONSTITUTION: A coating prepared by mixing a corona-proof member such as silicone carbide in resin is applied on a clothlike base material such as a glass base material, whereby the corona-proof tape 1 is formed. The front surface of the tape 1 is made half-cured, while the back surface 1b thereof is fully cured. The corona- proofing tape 1 thus prepared is wound round on the surface of a coil insulating layer 3 formed on a conductor 2 in such a manner that the half-cured front surface 1a touches the surface of the layer 3. This coil is contained in the slot of an iron core 4, impregnated with resin in vacuum and cured by heating, and thus the insulated coil is completed.