A welding apparatus of a stainless steel pipe comprises a welding torch jetting argon gas for shielding oxygen into a gap of the steel pipe rolled in a circular shape, while fusing any one of base metal and a welding material; and a gas supply pipe installed inside the steel pipe, the gas supply pipe having a jetting nozzle which jets argon gas at a rate of 4 to 20 l/min toward the inner surface of a welding bead portion, where welding has been already completed and which is positioned backwardly at a predetermined distance from a welding bead portion where welding is being performed by the welding torch.