A welding unit welds respective side panels pieces of a partly-finished product being conveyed by a conveying unit to each other along a conveying direction. The conveying unit conveys, when the respective side panel pieces are welded to each other by the welding unit, the partly-finished product in a state in which both side portions in the width direction of the portions of the respective side panel pieces to be welded are held for each set of the respective side panel pieces.