PURPOSE: To obtain a suitable cooling duct by a method wherein one side of a side wall and swollen portions at both ends of the wall are composed of one board and another side of the side wall is formed by connecting the same or board separated to the swollen portions and separating boards are provided between the side walls in the manufacturing process.

CONSTITUTION: Swollen tubular portions 16 are provided along the circular direction of a duct and at the top and bottom ends of a side wall 15 of the duct by forming a board 21 by a rolling process. Another side of the side wall 15 is formed by a board which is continuous to the bottom swollen portion 16 and is connected 22 to the board material of one side of the side wall 15 and a duct 21 is formed. Between both side walls, a plurality of narrow separating boards 23 are arranged vertically with a prescribed interval and the boards 23 are welded to one side of the side wall and is held by another side to form a path of a coolant. Outlets 17 are provided to the swollen portion 16 of the duct. By providing a cooling duct 9 formed in accordance with this constitution inside a foil winding coil of a transformer and the coolant is introduced, the coil is cooled efficiently.