To surely operate a welding torch by one drive source while keeping the prescribed relationship of the displacement of the welding torch in two directions by operating a torch swing device to swing the welding torch in the traveling direction of a truck.

Rotation of a drive motor is transmitted to an input shaft 9 through a power transmission mechanism 10. A casing 19 of a movable part is oscillated around the axis of the input shaft 9 by the rotation of the input shaft 9. Rotation is made around an idling gear 22, a planetary gear 21, and a sun gear 20 according to the oscillation of the casing 19, the planetary gear 21 is revolved in proportion to the oscillation of the casing 19, and a torch holder 4 to be turned in the same direction as the planetary gear 21 is also oscillated in proportion to the oscillation of the casing 19. A torch swing device is operated at each end of the traveling region of a traveling truck to swing the welding torch along the traveling direction of the truck The tip part of the welding torch can be projected outwardly of the forward and rear end faces in the traveling direction of the traveling truck.