PURPOSE: To prevent the occurrence of cracking of a welding starting end and to improve productivity by chamfering upper edges of a welding groove of a back bar of a joining device which welds beltlike metallic sheets on the back bar and rolls a weld zone.

CONSTITUTION: The beltlike metallic sheets 10 and 22 are superposed or abutted on each other on the back bar 31 of welding equipment and this superposed part or abutted part is arranged on the welding groove 31A of the back bar 31. In the joining device of the beltlike metallic sheets which welds the superposed part or abutted part by a welding torch 30 and rolls the beltlike metallic sheet ends 10A and 22A by a rolling mill, since the upper edges 31B and 31B of the welding groove 31A of the back bar 31 are chamfered, cracking does not occur on the welding starting end even when the weld zone of the beltlike metallic sheets 10 and 22 is welded at the welding speed of ≥5m/min.