PURPOSE: To perform welding-all-around with high accuracy by arranging a flange fitting jig, a cone member fitting jig and a welding torch on a first rotary table, on a second rotary table and between the first and second rotary tables respectively.

CONSTITUTION: In a state with the second rotary table 19 raised, a flange 8 is fitted to the flange fitting jig 17 on the first rotary table 16 and a cone member 6 is fitted to the cone member fitting jig 20 on the second rotary table 19. Next, the second rotary table 19 is lowered and the cone member 6 is abutted on the flange 8 while being rotated together. Next, the welding torch 22 is directed toward a weld zone of the cone member 6 and the flange 8 along the outside of the cone member 6 and a rotation axis 29 is driven to rotate by a rotation driving means 18. By this method, the cone member 6 and the flange 8 also rotate according to the first and second rotary tables 16 and 19 being driven to rotate together and the weld zone thereof is welded all around by the welding torch 22.