PURPOSE: To reduce iron loss and to improve magnetic characteristic by forming scratches with predetermined intervals in the longitudinal direction of wound amorphous magnetic thin belt.

CONSTITUTION: A thin belt 1 of amorphous magnetic metal is wound and it is subjected to heat processing under the N2 gas ambient. Next, for example, the internal end of toroidal core is extracted and the scratches 2 are set at the inside of thin belt 1, namely toward the center of winding at a right angle to the ribbon width with intervals of 1mm. The scratches 2 are formed at the inside in the shape of V at right angle to the longitudinal direction of thin belt 1 by drawing with a low pressure with a tungsten carbide needle. The winding is rewound to a toroidal core and is subjected to the casing. Such scratches 2 reduces a current loss and high frequency iron loss by segmentation of magnetic domain in the longitudinal direction. Simultaneously, pressure P is reduced in order to form the scratches 2 at a right angle to the longitudinal direction at the inside of winding surface of thin belt 1.