A condenser lens for collecting a laser beam (300) is disposed between the laser oscillator and the incident end surface of the optical fiber. The laser beam (300) is divided into a plurality of beams (303, 304). The power of the laser beam (304) is measured and maximized by adjusting the position of the condenser lens. The FFP of the laser beam (303) is measured and minimized by adjusting the position of the condenser lens. These adjusted positions are stored as the first and second lens positions. The FFP of the laser beam (303) is measured while the condenser lens is being moved between these positions so as to make the BPP not more than a predetermined value.