<p>A plurality of attachments (24) having springs (64) hold the outer plate (14) of a fuel tank (12), and an inner jig (20) positions the inner plate (16). The attachments (24) are attached to clamp arms (26) which is opened/closed by cylinders (28). When the overlap portion between the outer and inner plates (14, 16) is welded, the thermal deformation caused by welding is accommodated by the springs (64). The attachments (24) are spaced from the fuel tank (12) in the order in which the welding point is approached.</p>
