PURPOSE: To enable welding multiple welding positions on crossing lines with one welding action by aligning a center line of a laser beam with the crossing line of band plates and by positioning its focus point on the band plates to be welded together.

CONSTITUTION: A laser beam 25 is set so that a center line of the beam aligned with the crossing line of the band plates 7 and 8 and is collected by a collector lens 26, and the beam diameter is the smallest at a point close to a middle part 18 of an intersection. Also, a dimension (x) from a protrusion 20 of an upper intersection 17 to a protrusion 21 of the middle part 18 is shorter than the minimum value (d1) of a depth of focus (d) of a collected beam 27 collected by the lens 26, and the upper intersection 17 and the middle part 18 are placed at positions within the depth of focus (d1). When an irradiation of the laser beam is continued for a certain short period, a crossing point of band plates 7 and 8 is locally heated up by a high density energy of the beam 27 and consequently the upper intersection 17 and the middle part 18 are welded at the same time. As for a lower intersection, the same procedure can be applied by an upward irradiation of the beam 27 from a lower position.