To reduce the sagging of melted metal at the time of welding by setting a shifting quantity on an arc locus, operating an angle which is formed by a line connecting the position to an arc center and a line connecting the original position to the arc center and rotating a quantity equivalent to the angle an external shaft grasping a work.

An operational program is read from a memory, when a command is an arc interpolation, the central position of the arc is operated from the information for executing the arc interpolation. The shifting quantity of an arc starting position and the position are operated, a vector L which advances toward the original position 21 from the central position of the arc is operated. A vector M which advances toward the position 22 after shifting from the central position of the arc is operated, and the angle θ formed by the vectors L and M is obtained. The external shaft grasping the work is rotated by the angle θ, and the position of the vector M is moved to the vector L. When the rotational center of the external shaft does not coincide with the center of the work, correction is required.