PURPOSE: To shorten the impregnation time and thus to rationalize manufacture by winding into a coil insulating layer a spacer of polypropylene, Teflon or acetal resin, etc. having a large thermal expansion coefficient and releasing property, and impregnating the coil with an insulating synthetic resin.

CONSTITUTION: On a core 1 are wound 2W3 layers of a releasing paper, and as an inner insulating paper 2 an aromatic polyamide paper and a polyester nonwoven fabric are stack-wound thereupon. In this process, to form an inner insulating layer 4, tubular spacers 3 with 1mm outer diameter and 0.4mm inner diameter approx. of a material having a large thermal expansion coefficient and good releasing property are wound in the direction of the axis of the core 1 into the inner insulating paper 2, at 4W8 places by one turn with each spacer projected several cm from the end surface into which an insulating synthetic resin is to be impregnated. On this insulation layer 4 are wound one layer of a conductor, an insulating layer and the necessary layers of a blank coil. A leakage stop seal layer is formed on the coil circumference and one end surfaces. The coil is then put into an impregnating insulation synthetic resin tank wherer it is impregnated with an insulation resin. Then the coil is heated to harden the resin. This improves the impregnation speed and eliminates the use of a metal mold.