PURPOSE: To decrease titled iron losses at a low cost without producing burrs by projecting abrasive materials to a steel plate by way of an endless conveyor which has slits in the width direction of the steel plates at prescribed intervals and moves in synchronization with the traveling speed of the steel plate.

CONSTITUTION: Centrifugal forces are applied to granular materials 5 such as metallic particles, synthetic resin particles or the like by an impeller 4 and said materials are projected to the rear surface of a directional electrical steel plate 1 which is subjected to finish annealing. Then, the materials 5 are shielded by an endless conveyor 3 and are therefore projected only through the parts of the conveyor 3 where there are the slits 2, by which spotlike strains of fine linear shapes are applied to the plate 1. The materials 5 are bombarded to the surface of the steel plate not by such large forces by which the oxide films on the surface of the steel plate are dislodged but by the forces by which the surface of the steel plate is lightly tapped.