Technology scanning and irradiating grain-oriented electrical steel sheet moving in a rolling direction at a speed Vl with an elliptical spot formed by a laser beam in the width direction at Vc to improve the core loss of the grain-oriented electrical steel sheet, which adjusts the angle θs formed by the scan direction of the elliptical beam and the long axis of the ellipse based on the equation θs=tan−1(Vl/Vc) and which constructs the system for adjusting the θs by an optical system for laser irradiation comprised of a cylindrical lens telescope, scan mirror, and fθ lens arranged in that order, a system for changing the angle of the cylindrical lens telescope with respect to the scan direction of the scan mirror, and a system for changing the distance between lenses.