A double-wall structure consisting of an inner wall, an outer wall surrounding said inner wall and reinforcing ribs that connect said outer and inner walls is produced by a piercing welding method in which an electron beam is externally applied at right angles to the outer wall, such that the welded structure of the outer wall and each rib is composed of at least two piercing weld beads that are spaced apart by an unwelded area, with the sum of the widths of the weld beads being at least 25% of the rib width and the length of the unwelded area exterior to the root of each bead being no more than 20% of plate thickness. The distortion in welding is sufficiently reduced to enable precise assembling of the double-wall structure that can be fabricated efficiently and which can reasonably withstand the large electromagnetic force caused by plasma disruption.