Intending to apply an assembly-line-production, a mass production medium, to the assembly of flat blocks, it is essential for the plate joining work which is the beginning of the whole production to be applicable to the assembly-line-production. The conventional plates joining method is such that the right side of the plates is initially welded and the reverse side is also welded after turning them over. But the busy crane program in many cases prevents a smooth progress of work since the crane is not provided exclusively for this process. So, if such welding method, capable of welding two plates together in one process from the right side to the extreme reverse side, is available, it can eliminate such extra works as turnover and welding reverse side and can creates a smoother flow of material. Of various methods of automatic one side welding, the one we have developed is “Plate sliding type automatic one side welding” that the steel plates to be welded slide themselves through the welding point of a fixed welding machine, under neath which there is a piece of copper plate to keep pushing up the reverse side of the plates being welded. By this process, the plates are welded together from the extreme right side to the extreme reverse side. This particular welding method has required us so much study in the process of development especially regarding welding method itself and the holding copper plate system because of the fact that it is different in many respects from ordinary welding methods which hold steel plates to be welded still. As a result of our thorough study, we have successfully developed an automatic one side welding method capable of welding to the extreme reverse side of the plate in one process capable of handling plates, 10 mm to 40 mm in thickness and 10 m to 20 m in length. This new method is now being applied to our hull assembly work in factory.