PURPOSE: To execute a pipe welding by a shield gas of a small quantity, and to reduce a cost by making the shield gas flow along the inside peripheral surface of a pipe by using a nozzle for a shield gas.

CONSTITUTION: Pipes 5, 10 are butted at the seam part, a nozzle part 7 of a shield gas nozzle is inserted into one pipe 5, and a gas jet port 5a is opposed to a weld zone 8 of the seam part. In this state, a shield gas is jetted out of the gas jet port 6a, the shield gas is made to flow along the inside peripheral surface of the pipes 5, 10, and the weld zone 8 is welded by a welding torch 9.