PURPOSE: To enhance the productivity of iron cores are well as to perform an excellent crystallizing process by a method wherein an annealing process of the iron core consisting of an amorphous magnetic alloy thin plate and a partial crystallizing process are performed in one process.

CONSTITUTION: An iron core 11 is formed by winding a narrow-stripped amorphous magnetic alloy thin plate 12 rectangular form. Clamping plates 13 and 13 are arranged along the axial direction of the iron core on the outer and the inner circumferential surfaces of the yoke part of the iron core 11, and thin films 12 are tightly contacted each other by clamping and fixing with a nut 15 passing through screwed stud 14 at both ends of the clamping plates and by adding a compressing force from both sides in lamination thickness of the thin plates on each amorphous magnetic alloy thin plates 12 corresponding to the clamping plates. Under the above-mentioned condition, an annealing is performed by exciting high frequency, and the thin films are crystallized utilizing the high heating power of the amorphous magnetic alloy thin films of the part where compressing force of the iron core is applied.