PURPOSE: To prevent the outflow of thermosetting resin impregnated under the pressurized vacuum condition into the main insulating layer at the time of heat- hardening process by impregnating resin into the main insulating layer where the thermosetting resin easily outflows, then coating powder paint and hardening onlY the surface thereof under a low temperature.

CONSTITUTION: The main insulating layer 2 is formed by winding an insulating tape around a coil conductor 1. A thermosetting resin is pressurizingly impregnated under the vacuum condition to the layer 2. A powder paint is sprayed onto the layer 2 and thereby a sealed layer 3 where viscosity of impregnated resin is increased is formed. Thereafter, the layer is heated for 3W5hr at a low temperature of 60W100°C and only the surface is hardened. Thereafter, temperature is gradually increased, the surface is heated for about 10hr at a temperature of about 150°C in order to harden the layer down to the layer 2. Thereby, the outflow of resin is prevented and generation of void can also be auoided.