To provide a clamping tool for sheet welding with which a welding work can be performed with always suitable gap condition without wasting metallic blank having high heat conductivity and high cost.

The welding work is performed by supporting long backing members 2, 2 having rectangular cross section so as to be rotatable around a shaft with a first and a second backing member supporting blocks 3, 4 and positioning the butting portion of the sheets 14, 15 as the welding object on the gap Y formed in the interval between the long backing members 2, 2. Even in the case of developing shape variation and composition variation caused by repeating of the welding work to edge portions 2c, 2c of the long backing members 2, 2, since the other outer peripheral surface(one surface in four surfaces) under state of non-using at this time, can newly be realized as laying surfaces of the sheets 14, 15 as the welding object by rotating the long backing members 2, 2 at 90° pitch, a substantial durable term of the backing members 2, 2 can be extended to about four times in comparison with the conventional type of clamping tool for sheet welding realizing a one body type platy backing member. Thereby, the sure welding work can be performed on the proper gap condition.