PURPOSE: To realize insulation processing through impregnation of resin inside of a coil and provision of insulating layer at the outside thereof by heating a coil wound with a self fusing insulating lead wire and by effectuating powder resin which fuses at a temperature a little lower than the fusing temperature of a deposited layer while the fused condition of deposited layer is maintained.
CONSTITUTION: An electrical power is supplied to a coil wound with a self-fusing insulating lead wire, it is uniformly heated and a fused layer is sufficiently dissolved. Then, the heated coil is dipped into the powder resin which dissolves at a temperature a little lower than the dissolving temperature of fusing insulating layer, or such powder resin is sprayed thereon. At this time, the powder resin dis solves and is well impregnated into the coil, also deposited on the coil surface, forming sufficient insulating film without formation of boundary. Thickness of insulating film on the surface can be adjusted by the time for dipping into powder resin or spraying amount and spraying time and should be minimized within the range not deteriorating the insulation characteristic. According to these procedures, a coil having excellent insulation characteristic, mechanical strength, as well as good heat radiation characterstic can be obtained.