To obtain even welding joint seams for 3 or more strips without adding any structural and dimensional limit to a laser beam welding machine.

Work rolls 3a and 3b, which are bisected into the breathwise direction of a strip and installed with an opening gap each other at successively restricting positions of adjoining 2 strip edges, a retaining apparatus consisting of a pair of backup rolls 5 and 6, which clamp the work rolls to the strip, and a joint unit consisting of a welding head of a laser beam welding machine are installed to a circumferential direction of a position fix roll 13 with clamping the strip. Thus, the laser beam is able to pierce a gap between the pair of backup rolls 5 and 6 and a gap 4 between the divided work rolls 3a and 3b.