PURPOSE: To stabilize a profile welding by providing a locking mechanism on the guide arm of a truck and by arranging the sensor for detecting vertical member with its projection by the prescribed distance.

CONSTITUTION: The rear end of the guide arm 15 which is equipped with a longitudinal plate profiling roller is borne at the truck turning center position. A hollow 63 is provided in the medium of the arm 15 and the locking piece 64 which fits by being driven with a solenoid 65 at the profiling travel time is arranged as well. Moreover, the sensor which detects the vertical member 11, 12 at the front of the truck is arranged by its projection by the prescribed distance so that the sensor can detect the vertical member 11, 12 with the truck center being before the prescribed distance of the vertical member 11, 12 at the front part. The locking piece 64 is removed from the hollow 63 by energizing the solenoid 65 at the crossing part, then the arm 15 is locked again by starting a straight line profiling after the truck spin. With this method, the locking of the guide arm 15 is ensured and the horizontal fillet profiling welding is stabilized.