PURPOSE: To increase a productivity and magnetic characteristics of a core by forming the core in such a structure that nearly rectangular blocks which are made by winding thin belts in the stacking direction in a specified thickness and which have specified outer diameters are substantially adhered tightly to each other.

CONSTITUTION: An amorphous magnetic alloy-made thin belt 2 is wound from a material hoop 4 around an iron block 3 which is nearly square and has a specified size. By this method, a set of nearly square-shaped three iron blocks 3a, 3b, and 3c, each of which has curved sections only at four corners, are adhered tightly at linear sections. At the four corners, nearly crescent-shaped spaces 7 are formed between the iron blocks 3a, 3b, and 3c. By this, over- tightening is prevented at the corners of a core and thereby the transmission of excess compressive force to the inside of a core window can be prevented and the deterioration in characteristics of the amorphous magnetic alloy-made thin belts can be remarkably lessened.