The front and rear surfaces of plates to be butt-welded are slidably held by front and rear backing shoes, respectively, for defining a welding space in a narrow groove formed by the edges of the plates, and the edges are melted by a plasma arc generating torch, while a plurality of consumable electrodes are fed into the welding space are molten by arcs generated therebetween. The backing shoes, the torch, and the consumable electrodes are moved along the groove to obtain a butt-weld of high quality at high efficiency. In addition, the electrode of the torch has an elongated configuration in section at its end portion, and its restraining nozzle surrounding the electrode has a nozzle opening which also has an elongated configuration in correspondence to the end portion of the electrode. The longitudinal axes of these configurations are aligned with each other to effectively utilize the energy of a plasma arc generated by the torch.