PURPOSE: To satisfactorily weld a butting part by fixing one pair of connected clamp heads to the butting part of a welding pipe from inside and filling up the groove part for welding with the shield gas from the groove in the one side of the clamp head.

CONSTITUTION: A jig is inserted in the pipe 13 to be welded, and after positioning a first clamp head 1 so as to come to the pipe end 13a of the pipe 13 to be welded, a second clamp head 2 is protruded to outside with an air cylinder 7 and a ring mechanism 9 and the jig is fixed to inside of the pipe 13 to be welded. At this time, adjustment is executed so that the pipe end 13a is positioned outside of the groove 10 in the first clamp head 1. Successively, the welding pipe 14 is inserted from the first clamp head 1 side and under the condition of butting the pipes 13 and 14, the first clamp head 1 is protruded outside and the welding pipe 14 and the jig are fixed. Under this condition, the gas for shielding is fed in the groove 10 and fills in the groove 10 to complete the back shield at the groove part for welding.