To provide a backing for welding which can improve a working efficiency at the time of fitting the backing, and to provide a welding method.

In the backing 1 used for butt-welding a square column 2 to a diaphragm steel sheet, recessed parts 6 are formed in accordance with weld bead faces 2c formed at the insides of the square column 2. The square column 2 and the backing 1 are welded in such a manner that the recessed parts 6 in the backing 1 are combined with the weld bead faces 2c, and, at least in the recessed parts 6, the square column 2 and the diaphragm steel sheet are temporarily welded. Thereafter, the square column 2 and the diaphragm steel sheet are welded.