PURPOSE: To facilitate the assembling work of a core as well as to reduce an iron loss without making a scrap by a method wherein the lower yoke side is constituted by laminating a core material pieces one by one, and the upper yoke side is constituted by laminating set by set of a set consisting of a plurality core material pieces of almost same shape.

CONSTITUTION: Core material pieces are cut in the order of (a), (b), (c), and (f) successively. Pertaining to the lamination work of the core material pieces, as a lamination pattern wherein a set of two core material pieces of the same shape and the same dimensions are repeatedly laminated at each position of the upper yoke is used, the laminating method same as that heretofore in use can be adopted. To be more precise, a set of core material pieces 4a and 4b of the upper yoke 4 which are cut at (a) and (b), a set of core material pieces 5a and 5b of the upper yoke 5, a set of core material pieces 4c and 4f of the upper yoke 4 which are cut at (c) and (f), and core material pieces 5c and 5f of the upper yoke 5 are laminated respectively. On the other hand, pertaining to the lower yokes 6 and 7, the core material pieces of the lower yoke 6 or 7 on the same position are differentiated in shape of each sheet in the direction of lamination. Thus, on the connecting parts of the outer leg 1 and the lower yoke 6, and the outer leg 2 and the lower yoke 7, a connecting method wherein core material pieces are overlapped one by one is used, and the iron loss at this part can be reduced.