To provide a backing strip for welding a column, with which a chipping off work for a tack-welded zone is eliminated and the deterioration of the weld strength due to a weld defect in the tack-welded zone is prevented.

The backing strip 10 is fixed in the inner face of the end part of the column 1 and closes the gap 3 of a groove formed between both base members 1 and 2 from the inside of the column 1 when the end part of the column 1 is joined by welding to the side face of a diaphragm 2. A stepped part 14 which is stepped down from the thickness of the tack-welded zone 7 is formed in the vicinity of a position corresponding to the end part of the groove of the column 1, and a bottom surface 15 of the groove, which extends from the stepped part 14 to the side edge on the side of the diaphragm 2 substantially in parallel with a fixed face 12 and closes the gap 3 of the groove from the inside is formed at a position lower than the fixed face 12 with the column 1.