PURPOSE: To rationalize manufacture by sealing the circumference and end parts of a blank coil wound on a core having a large expansion coefficient, drying the coil while being heated to expand the inner dieaeter of the coil and injecting resin into the core-to-coil space produced after being cooled.

CONSTITUTION: A coil 2 and an insulating layer are stack-wound on a plastic core 1 having a large thermal expansion coefficient, and an outer seal layer 3 is formed at the coil circumference and both end surfaces. The coil 2 wound on the core 1 is dried at about 130°C to harden the seal layer and at the same time to expand the core 1 which is then cooled to contract it, causing a space 5 to be formed between the coil 2 and the core. A leakage stop seal 4 is provided to a part of the space 5. Under this condition the coil 2 and the core 1 are put into an insulating synthetic resin tank where they are impregnated with an insulating synthetic resin under pressure. Then, they are heated so that the resin can be hardened. This constitution eliminates the use of a metal mold, reduces the impregnation time, and thus rationalizes manufacture.