PURPOSE:To make an annealing temperature uniform by previously forming a dielectric layer between the wound layers of an alloy thin belt and on the surface of an iron core and by heating applying voltage to the dielectiric layer from electrodes provided on both the inside and the outside surfaces and the sides of the iron core in annealing a wound core wound with an amorphous magnetic alloy thin belt. CONSTITUTION:A cirular wound core 11 is wound with an amorphous alloy thin belt 12. The wound core 11 is removed distortion and annealed in magnetic field. In this construction, a dielectric layer 13 is formed in all circular directions between the wound layers of the alloy thin film 12 and on the external and the internal sufaces an on both the side end surfaces of the iron core 11. In this case, such a material as epoxy resin which has great dielectric dissipation factor against high frequency voltage is used for the dielectric layer 13 and circular ring electrodes 14 on the external and the internal surfaces and circular plate electrodes 15 on both the end surfaces of the iron core 11 are provided. Then, the iron core 11 is put in an annealing furnance, high frequency voltage is applied to the electrodes 14 and 15, simultaneously, magnetic field is generated from a coil around the iron core 11 and the iron core 11 is heated at approx. 400 deg.C.