PURPOSE: To eliminate the installation of rails and to weld continuously the entire circumference of a tank, by mounting the apparatus, materials, etc. necessary for welding on a carriage which travels along the shell of the tank and welding the entire circumference of the tank.

CONSTITUTION: A carriage frame 12 is run along the shell 10 of a tank by means of traveling rolls 13, 14 and a guide roll 17 so as to face to the outside plate 10A of the tank to be welded. The torch 3 on the frame 12 is moved upward and downward so as to face to the butt parts of the shell 10 and the plate 10A. A welding wire, a cable for arc and a shielding gas are supplied to the torch 3 and the butt parts are continuously welded. When the welding of the plate 10A is completed, the next outside plate 10B of the tank is placed along the shell 10 and the similar welding operation is repeated. When one round of welding the tank is completed in the above-mentioned way, the shell 10 is lifted and the outside plate 10B in the succeeding stage is placed under the shell 10 and is welded.