PURPOSE: To improve the insulating characteristics of an amorphous magnetic core and to enhance the occupying rate of the core by laminating thin amorphous magnetic plates formed with oxidized film on the surface by electrolysis.

CONSTITUTION: Insulating film is formed on the surface of a thin amorphous plate by electrolysis by utilizing the conductivity of the amorphous magnetic material. The thickness of the film by the electrolysis can be arbitrarily selected by controlling the current flow rate. To raise the efficiency of forming the insulating film, ribbon- shaped amorphous magnetic material (Fe, etc.) is used as an anode, and the insulating film may be formed continuously by infinitesimally moving it in the electrolyte.