PURPOSE: To improve a weld line follow-up property of a welding torch under the condition that stability of an arc is low, by detecting a quantity of electricity corresponding to the arc of the welding torch, and impressing the prescribed correcting quantity to a welding torch movement control device in accordance with a deviation to a reference quantity.

CONSTITUTION: When a welding torch 7 approaches from a desired distance or is separated in the direction of the Y axis against a weld line WL, a difference is generated between a set value of a setting device 25 and an integral value of a Y axis integrator 26, and a comparator 29 detects a deviation. Subsequently, basing on the deviation, a Y axis servo-motor YS adjusts a distance in the direction of the Y axis of the welding torch 7 and sets the deviation to zero. Also, when the center of a weaving pattern is shifted to one of the left or the right against the weld line WL, a difference is generated in an integral value between left and right integrators 27, 28. As a result, a comparator 30 detects a deviation of both of them, driving of an X axis servo-motor XS is controlled by a detecting value, and the weld line WL coincides with the center position of the weaving pattern.