One-side automatic welding, a method of butt welding from one side only, has been successfully developed for thick plate of over 20 mm. As to thin plate of about 10 mm, however, there existed many difficult problems. Major difficulties were on the distortion of plate and inaccuracy of edge preparation. Some investigations has been carried out in our yard to solve this problems, and the new method has been established. In this method, thin flux layer is sandwitched between plate and copper strip, and submerged arc welding is used. Certainty of back-side bead formation and soudness of welded joint are fully satisfactory. This procedure has successfully been used in an assembling line of ship construction in the yard.