PURPOSE: To facilitate tightening a gapped core effectively without degrading insulation characteristics by a method wherein upper and lower clamps and side leg application plates are combined to constitute a framework and crossbars abutted against yokes are tightened with the framework.

CONSTITUTION: A plurality of block cores 2 which are composed of laminated silicon steel bands are piled with gap members 3 made of insulating material between each other so as to form magnetic gaps to constitute a gapped core 21 and a winding 4 is applied to the gapped core 21. Then upper and lower yokes 1 and 14, which are composed of laminated silicon steel bands tightened with clamps 8 and 13, are contacted with both the ends of the gapped core 21 and the upper and lower clamps 8 and 13 are made to cross side legs 5 which are composed of silicon steel bands laminated in the same way and pairs of side leg application plates 11 holding and tightening the laminated steel bands and the side leg application plates 11 are made to be continuous with the upper and lower clamps 8 and 13 and, further, crossbars 6 and 12 are provided outside the upper and lower yokes 11 and 14 and the crossbars 6 and 12 and the upper clamp 8 are linked by bolts 7a and nuts 7b. With this constitution, a core tightening construction which facilitates tightening the gapped core effectively and, moreover, does not degrade the insulation characteristics can be realized.