A method for optical axis alignment between a laser oscillator and an optical fiber by using a laser light evaluator, the laser light evaluator including a light-collecting unit, and a light-receiving unit having a light-receiving surface configured to receive laser light emitted from the laser oscillator and guided by the optical fiber and collected by the light-collecting unit, the light-collecting unit having an fθ lens configured to modify a shape of the laser light such that a radiation angle distribution of the laser light is converted into a position distribution, a field lens configured to transmit the laser light with the shape modified by the fθ lens and to change a direction of travel of the laser light, and an image-forming lens configured to collect the laser light onto the light-receiving surface of the light-receiving unit, the method comprising: receiving laser light on the light-receiving surface of the light-receiving unit, the laser light having been emitted from the laser oscillator and guided by the optical fiber via an axis alignment mechanism situated between the laser oscillator and the optical fiber; and aligning an axis of the laser oscillator with an axis of the optical fiber using the axis alignment mechanism based on a light intensity distribution of the laser light received on the light-receiving surface of the light-receiving unit, the aligning being performed while monitoring a numerical aperture of the laser light radiated from the optical fiber until the axis of the laser oscillator and the axis of the optical fiber are aligned such that the numerical aperture reaches a minimum value, the numerical aperture being derived from at least one of following formulas indicating an intensity profile in an X direction and an intensity profile in a Y direction obtained from the light intensity distribution: NA = sin x / f ′, and NA = sin y / f ′ where NA represents the numerical aperture, x represents a length between a first position and a second position, the first position being where an intensity has a maximum in the intensity profile in the X direction, the second position being where the intensity is 1/e 2 of the maximum in the intensity profile in the X direction, y represents a length between a third position and a fourth position, the third position being where an intensity has a maximum in the intensity profile in the Y direction, the fourth position being where the intensity is 1/e 2 of the maximum in the intensity profile in the Y direction, and f' represents a focal length determined from the optical system of the light-collecting unit. receiving laser light on the light-receiving surface of the light-receiving unit, the laser light having been emitted from the laser oscillator and guided by the optical fiber via an axis alignment mechanism situated between the laser oscillator and the optical fiber; and aligning an axis of the laser oscillator with an axis of the optical fiber using the axis alignment mechanism based on a light intensity distribution of the laser light received on the light-receiving surface of the light-receiving unit, the aligning being performed while monitoring a numerical aperture of the laser light radiated from the optical fiber until the axis of the laser oscillator and the axis of the optical fiber are aligned such that the numerical aperture reaches a minimum value, the numerical aperture being derived from at least one of following formulas indicating an intensity profile in an X direction and an intensity profile in a Y direction obtained from the light intensity distribution: NA = sin x / f ′, and NA = sin y / f ′ where NA represents the numerical aperture, x represents a length between a first position and a second position, the first position being where an intensity has a maximum in the intensity profile in the X direction, the second position being where the intensity is 1/e 2 of the maximum in the intensity profile in the X direction, y represents a length between a third position and a fourth position, the third position being where an intensity has a maximum in the intensity profile in the Y direction, the fourth position being where the intensity is 1/e 2 of the maximum in the intensity profile in the Y direction, and f' represents a focal length determined from the optical system of the light-collecting unit. NA represents the numerical aperture, x represents a length between a first position and a second position, the first position being where an intensity has a maximum in the intensity profile in the X direction, the second position being where the intensity is 1/e 2 of the maximum in the intensity profile in the X direction, y represents a length between a third position and a fourth position, the third position being where an intensity has a maximum in the intensity profile in the Y direction, the fourth position being where the intensity is 1/e 2 of the maximum in the intensity profile in the Y direction, and f' represents a focal length determined from the optical system of the light-collecting unit.