PURPOSE: To manufacture an iron core superior in low loss characteristic, by bringing a metallic short-circuit sheet into contact with piled layer surface of the iron core composed of amorphous magnetic alloy sheet, exciting said core by AC current to heat and anneal it.

CONSTITUTION: The band-shaped amorphous magnetic alloy sheet 2 is wound continuously to form a rectangular wound core 1. The short-circuit sheets 3 (sheet material of highly conductive metal material such as Cu) are brought into contact with both piled layer surfaces of the core 1 over respective whole surfaces. Next, a coil 4 for high frequency excitation is wound at the leg part of the core 1, and if high frequency (about 2W4kHz AC current) is conducted to the coil 4 in high frequency annealing tank of inert gas atmosphere, magnetic flux is caused in the core 1, and eddy current flows therethrough. Thus, Joule heat is generated due to power loss, the core 1 itself is heated by internal heat generation, and the temp. is rised. Electricity conduction is stopped after holding said core at a prescribed temp., said core is cooled while forming magnetic field by conducting DC current to the coil 4 and strain removal annealing of the core 1 is carried out.