A method of detecting position data in arc welding by monitoring the shape of light of welding arc and/or the position thereof. That is, in effecting the arc welding along the position of the beveling while detecting the position of the beveling, the shape of the arc light is imaged by an imaging device from above the joints of the members that are to be welded together, and the shape of the arc light thus imaged is converted into binary values. The shape and/or the position of the arc light converted into binary values are monitored, thus making it possible to detect the position of beveling, height of welding torch, starting point of welding, ending point of welding, corner point of the outer track, corner point of the inner track, work contact point of the welding wire and the gap of the beveling.