PURPOSE: To improve the productivity of welding work and the quality and reliability of weld zone in a welded structure having a circumferential joint consisting of a linear part and a curved part by welding the whole circumference of the weld line continuously and automatically by combining a work rotating device and a weld copying device.

CONSTITUTION: The work rotating device 5 that attaches a work 9 to left and right face plates 1 and rotates it is provided with a rotation speed detector 4 of the work 9 and a welding torch 11 that belongs to welding executing devices at the tip, and a manipulator 6 that operates welding of the work 9 is also provided. The welding torch 11 is connected to the manipulator 6 by upper and lower sliding devices 12 through the weld copying device 12, and the weld copying device 12 is provided with an angle of inclination detector 4 of the work 9 and a welding speed detector 15. By such constitution, weld lines of the whole circumference of the work 9 are welded automatically, and the quality and reliability of the weld zone and the productivity are improved.