A laser welding method is provided, by which the suppression of porosity is obtained even when a laser beam is not spun. Laser beam 1 is irradiated onto object W to be welded, to weld it while the focus position of the beam is vibrated in the optical axis direction. The beam focus position can be vibrated in the optical axis direction by using curvature variable mirror 7 in which the curvature of the reflecting surface can be controlled. The curvature variable mirror 7 having a structure in which a reflecting plate is transformed by a piezoactuator, and which can be driven at a high speed, is preferable as it allows the increase of the frequency of the vibration in the optical axis direction.