PURPOSE: To force the minute region in the surface layer of a grain-oriented silicon steel sheet inward and to reduce the iron loss by irradiating the sheet with an electron beam generated with a high voltage and a low current through an atmosphere with the pressure gradually increased from a high vacuum to atmospheric pressure.

CONSTITUTION: An electron beam generator is formed with a high-voltage insulator 1 consisting of an insulating material, an electron gun 2 with a high voltage and a low current and an anode 4 for the gun 2, and an electron beam 3 having a small diameter and a great infiltration depth is generated. The electron beam generator is kept at a high vacuum by a column valve 5, and the scanning of the focus of the beam 3 is adjusted by an electro-magnetic lens 6 and a deflecting coil 7. Meanwhile, a bank 8 of differential-pressure chambers is arranged in the traveling direction of the beam 3. The pressure is gradually increased from a high vacuum to atmospheric pressure by the chambers 8-1 to 8-4, and the grain oriented silicon steel sheet 10 is irradiated with the beam 3 through a passage 9. Consequently, the magnetic domain can be finely divided by the beam 3 in the atmosphere, and the iron loss of the sheet 10 is advantageously reduced.