PURPOSE: To enable automatic welding until a welding torch arrives at the deepest part of a corner wall surface by moving a welding lever by a turning lever while restricting the welding lever by a specified curve so that the locus at one end of the welding lever holding the welding torch attains a prescribed shape.

CONSTITUTION: A welding lver 3 holds a welding torch 1 at one end thereof and the other end 4 thereof is pivotally pinned 7 to the top end 6 of a swiveling lever 5. The bottom end 9 of a leg 8 suspended to the bottom surface of the lever 3 fits into the groove 11 provided atop a guide plate 10 and is forced to move along the groove 11. The lever 5 is fitted and fixed to the shaft end of a deceleration motor 12 and is swiveled in a specified direction, for example, a direction P. The shape of the groove is given as a specified curve and consists of grooves 11a, 11b up to 0W180°, 180W360° positions of the lever 5 which are connected to each other in the position corresponding to the 180° point. The locus at which one end of the lever 3 moves is thus made into, for example, the straight line shown in the figure and the linear welding along a weld line W is made possible.