A method for alignment between a laser oscillator and an optical fiber to be connected to the laser oscillator, the method comprising: placing a condenser lens and a lens adjustment mechanism between a light emission part of the laser oscillator and an incident end of the optical fiber, the condenser lens being configured to collect a laser beam emitted from the laser oscillator, and the lens adjustment mechanism being configured to adjust a position of the condenser lens, the method further comprising: a measurement step of dividing the laser beam emitted from an emission end of the optical fiber into a plurality of laser beams including at least a first laser beam and a second laser beam, and measuring a power of the first laser beam and a far field pattern FFP of the second laser beam; a first adjustment step of adjusting the position of the condenser lens such that the measured power of the first laser beam is maximum, and storing the position of the condenser lens as a first lens position; a second adjustment step of calculating a beam parameter product BPP of the second laser beam from the FFP of the second laser beam measured in the measurement step, adjusting the position of the condenser lens such that the BPP is minimum, and storing the position of the condenser lens as a second lens position; a third adjustment step of measuring an FFP of the second laser beam and calculating therefrom a BPP, each time the condenser lens is successively moved by a predetermined spacing from the second lens position in the direction of the first lens position; and a fourth adjustment step of when the position of the condenser lens has reached the first lens position, and the BPP calculated from the FFP of the second laser beam measured in the third adjustment step is not less than a predetermined value, fixing the condenser lens at the position immediately before the position where the BPP has the predetermined value, as the final position of the condenser lens and when the BPP is less than the predetermined value, fixing, as the final position of the condenser lens, the first lens position. a measurement step of dividing the laser beam emitted from an emission end of the optical fiber into a plurality of laser beams including at least a first laser beam and a second laser beam, and measuring a power of the first laser beam and a far field pattern FFP of the second laser beam; dividing the laser beam emitted from an emission end of the optical fiber into a plurality of laser beams including at least a first laser beam and a second laser beam, and measuring a power of the first laser beam and a far field pattern FFP of the second laser beam; a first adjustment step of adjusting the position of the condenser lens such that the measured power of the first laser beam is maximum, and storing the position of the condenser lens as a first lens position; adjusting the position of the condenser lens such that the measured power of the first laser beam is maximum, and storing the position of the condenser lens as a first lens position; a second adjustment step of calculating a beam parameter product BPP of the second laser beam from the FFP of the second laser beam measured in the measurement step, adjusting the position of the condenser lens such that the BPP is minimum, and storing the position of the condenser lens as a second lens position; calculating a beam parameter product BPP of the second laser beam from the FFP of the second laser beam measured in the measurement step, adjusting the position of the condenser lens such that the BPP is minimum, and storing the position of the condenser lens as a second lens position; a third adjustment step of measuring an FFP of the second laser beam and calculating therefrom a BPP, each time the condenser lens is successively moved by a predetermined spacing from the second lens position in the direction of the first lens position; and a fourth adjustment step of when the position of the condenser lens has reached the first lens position, and the BPP calculated from the FFP of the second laser beam measured in the third adjustment step is not less than a predetermined value, fixing the condenser lens at the position immediately before the position where the BPP has the predetermined value, as the final position of the condenser lens and when the position of the condenser lens has reached the first lens position, and the BPP calculated from the FFP of the second laser beam measured in the third adjustment step is not less than a predetermined value, fixing the condenser lens at the position immediately before the position where the BPP has the predetermined value, as the final position of the condenser lens and when the BPP is less than the predetermined value, fixing, as the final position of the condenser lens, the first lens position.