PURPOSE: To increase the cooling efficiency as well as to simplify the manufacturing process of the titled transformer by a method wherein a circular arc cooling duct having the circumferential angle of 180° or below is used, metal fins having low rigidity at both ends are attached, and they are tightly contacted to a leaf winding.

CONSTITUTION: A cooling duct 29 is provided at the angle of the circumference of 180 degrees or less, and a metal fin 30 of excellent coefficient of thermal conductivity is attached at both ends in circumferential direction. The fin 30 is to be formed in the thickness same as that of the leaf winding or less than that, namely, 0.1W0.5mm or thereabout for the purpose of obtaining a tight contact easily by reducing rigidity when it is wound on the leaf winding. This fin 30 fixed on the welded part 31 on the inner diameter side of the end part of the cooling duct 29. As the cooling effect of the cooling duct 29 is spread all over the fin 30 and the entire leaf winding which is in contact with the fin 30 is cooled, the cooling effect in the degree equivallent when the cooling duct is provided on the whole circumference can be obtained.