PURPOSE: To track a moving joint with a laser beam and to perform welding by deciding the centerline location of the joint with a laser profiler attached to a focusing head, before the irradiation of a welding beam, and holding the welding beam focal point at the prescribed centerline location of the joint.

CONSTITUTION: A detecting head 20 of a laser profiler is mounted to a focusing head 68 for projecting a laser beam 12 to a moving elongated joint 16. Then, the profile data of the joint 16 is formed by inspecting and processing the profile with the detecting head 20 prior to the projection of the laser beam 12. After that, this profile data is computer-processed, deciding the centerline location of the joint 16, and converting it to a tracking coordinate. Thus, by using the converted location data, the laser beam 12 faithfully tracks the joint 16 for welding.