PURPOSE: To achieve a transformer characterized by low leakage flux and low loss and having inexpensive production cost and qualititive stability by a method wherein U-shaped cores are combined with the windows of an EI- shaped or EE-shaped laminated core and are arranged orthogonally with the lamination direction of an I-shaped core thereby to reduce reluctance.

CONSTITUTION: U-shaped support cores 13 and 14 are arranged in the window portions 12 of a E+I-shaped core 11 constructed by abutting an E-shaped core 9 and an I-shaped core 10. Then, support cores 13 and 14 are arranged so that the two end faces of them are orthogonal with the I-shaped core 10 and coincident with the window diameter (a) and the window width of the window portion 12 of the E-shaped core 9, thereby to form a rectangle-shaped magnetic path. For the support cores 13 and 14, a material of higher permeability than the E-shaped core 9 and the I-shaped core 10 is chosen, and the quality of the material and the number of laminations may be made variable. Such support cores 13 and 14 can easily be manufactured by punching and bending processes with no material loss and inexpensively, and they prevent an air gap from appearing at the joint where the U-shaped support cores 13 and 14 are abutted against the I-shaped core 10.