PURPOSE: To execute high-speed scanning regardless of a scanning width by reflecting a laser beam by a rotary reflecting mirror, the reflecting mirror surface of which inclines with the plane orthogonal with a revolving shaft and synchronizing the irradiation timing of the above-mentioned laser beam with the rotation of the rotary reflecting mirror.

CONSTITUTION: The reflected light advances along an optical axis L2 shifted by<θ from a reference optical axis L1 when the reflecting mirror surface of the rotary reflecting mirror 1 is intersected orthogonally with the revolving shaft when the laser beam is emitted while the rotary reflecting mirror 1 is held stopped. The position deviated by l from the point intersected with the above-mentioned reference optical axis L1 is irradiated with the spot of the laser beam on a material M to be irradiated. The reflected light draws the rotating locus of the generator in the circular conical shape having a vertical angle at <2θ when a motor 2 is rotated. The surface of the material M to be irradiated is so irradiated with this laser beam so as to draw the circular locus of a radius l centering at the point intersected with the reference optical axis. A condenser lens 5 is set in the position forming a focus just before the material M to be irradiated and covers the reflected light passing all of the optical axis L2 shifted by <θ from the reference optical axis L1.