In order to develop an arc sensor using the mechanically rotating arc, a theoretical modeling and experiments were carried out to investigate the sensitivity and the accuracy of the "rotating arc sensor". The automatic seam tracking sensor was successfully developed for steel and Al alloy welding. The increase of the sensitivity and the responsiveness guaranteed an accurate seam tracking, and the way in which the welding and rotating parameters affect the accuracy of seam tracking was clarified from the experimental results. With the increase of responsiveness, the rotating arc sensor was able to be effectively applied to detecting the end-point due to the decrease of the influence of molten pool shape when the welding arc is on the preceding half circle.