PURPOSE: To scan a laser beam at a high speed with high accuracy and to permit uniform laser beam processing by repeatedly scanning the laser beam by using an acoustooptic element to form the quasi-linear beam having a specified width.

CONSTITUTION: The laser beam B irradiated from a CWAr+ laser 1 is conducted by a reflecting mirror 2 to the acoustooptic element 3. The beam B is scanned in an x-axis direction at the prescribed repetitive frequency and is made quasi- linear by the element 3 which receives the control signal from a control device 7. A sample table 5 is swept in a y-axis direction (perpendicular to the plane of the figure) while such quasi-linear laser beam is condensed by a condenser lens 4 and is irradiated to a silicon wafer 6 after ion implantation which is the work piece provided on the table 5.