PURPOSE: To improve the yield of a raw material by a method wherein, after rectangular thin silicon steel plates have been laminated and have been bent in the longitudinal direction so as to be U-shaped, two pieces of this U-shaped core having a saw-toothed part at least at one end are combined in such a way that a sawtooth-like air gap can be formed between the facing parts.

CONSTITUTION: After several sets of rectangular thin silicon steel plates, each having a length of l1,l2, l3 ..., have been laminated and combined by making one end (d) flat, this laminate is bent in the longitudinal direction so as to be U-shaped so that a U-shaped core 5 having a saw-toothed part at the other end(c)can be formed. Two pieces of this cord 5 are combined face to face with each other so that an air gap(g)can be produced in such a way that several triangles are formed between the facing parts. Through this constitution, because a punching process is not required, it is possible to achieve a yield rate of 100 %.