PURPOSE: To suppress swell of an insulating tube and secure a good vertical oil path by extending a duct piece for a horizontal oil path and contacting it with a multi- layered paper insulating tube.

CONSTITUTION: Both ends of a duct piece plate 2, which is inserted among a plurality of coils 1 and defines a horizontal path between them, are extended radially. These extentions 2a and 2b are contacted with both an inner insulating tube 4 and an outer multilayered paper insulating tube 5. Therefore, swell to the inside of the outer multilayered paper insulating tube 5 is suppressed with the outer extention end 2b. An outer vertical oil path 7 is secured fairly.