Herein disclosed is a method of detecting information on an arc welding position by monitoring the shape and/or position of an welding arc. When in the arc welding along a welding groove position while locating the same, the shape of the arc is picked up from above the joint portion of a member to be welded by means of an image pickup device, and the arc shape picked up is binary-coded. This binary arc shape and/or position are monitored to detect the welding groove position, the height of the welding torch, the welding start point, the welding end point, the external corner point, the internal corner point, the contact point of the welding wire with the work, and the welding groove gap.