PURPOSE: To reduce the deterioration in strength of peripheral base metal by butting a pipe to be welded against an installed pipe by a clamping device and welding the butt part from the inside and outside of the pipes and lessening a groove cross-sectional area and reducing the quantity of deposited metal.

CONSTITUTION: Inside welding equipment 1 is charged in the installed pipe 2 in advance and in addition, the clamping device 24 is fixed on the outside thereof. The pipe 3 to be welded is suspended and its one end is brought close to the end face of the installed pipe 2. In that state, the pipe 3 to be welded is held between clamp shoes 32 and further, the pipe ends are butted against each other and a joining part of the pipe ends is held between clamp shoes 37. Next, a graveling motor 5 of the inside welding equipment 1 is actuated and the inside welding equipment is moved to the prescribed position and the rotation driving motor 5 is actuated and a welding source is turned on to perform the inside welding. Next, the clamp 24 is removed and outside welding equipment 38 is fixed on the installed pipe 2 side and a groove part whose inside has been already welded is welded from the outside by a welding torch 45 fitted to a rotational ring 41 of the welding equipment 38.