PURPOSE: To copy a groove part accurately and to weld a branch pipe satisfactorily by storing the positions detected with a sensor and positioning a succeeding torch to correct positions.

CONSTITUTION: When a pinion 18 is rotated by a motor 19, a carriage 15 starts rotating around the axial line of a branch pipe 2. A sensor 22 detects the groove part. In accordance with the signal thereof, driving motors 23a, 24a for a vertical slider 23 and a lateral slider 24 run and the movement extents thereof are detected with detectors 23b, 24b, then the position of the groove part with respect to the rotating angle of the carriage 15 is stored in a controller. On arriving of a welding torch 10 at the detection start position of the sensor 22, a vertical slider 13 and a lateral slider 14 move is accordance with the signals of the detectors 23b, 24b, and the torch 10 copies the groove part accurately in accordance with the change in the welding groove. According to the rotating angle of the carriage 15, the torch welds the butt parts of a main pipe 1 and the branch pipe 2 in accordance with the control signal.