PURPOSE: To avoid cracking and prevent coil strength from weakening by a method wherein fillers made of inorganic material and organic resin are filled into large gaps formed by the corners of sheathing tubes and a like and are buried in insulating material by vacuum impregnation.

CONSTITUTION: A filler 5 which is preformed to fit a large gap formed by corners of sheathing tubes 2 is filled into the gap in the coiling process. With this process the large gaps are almost fully filled by the fillers 5 with high strength and the small gaps are filled by insulating material 3 by vacuum impregnation. With this constitution, the gaps of the coil are not filled by the continuous insulating material 3. So cracking which covers all area of the gap hardly occurs and the central part is secured by a high strength filler 5 so that a little inferiority of the inslating material 3 in the small gaps formed by the filler 5 and the sheathing tube 2 will not cause any mechanical problem. By selecting the same insulating material 3 as the organic resin of the filler 5, changing of thd dimensions caused by thermal contraction when the coil is cooled down is reduced. This also reduces the generation of cracking.