Partly-finished products having side panel pieces are supplied intermittently to places between welding surfaces of a horn and a rotating anvil. Thus, the side pieces of the partly-finished products, are welded continuously to each other with a predetermined pattern. The welding surfaces of the anvil are provided over an entire circumference about a rotating shaft so that the interval between the welding surfaces of the anvil and the horns is maintained at an interval at which the respective side panel pieces are continuously weldable to each other with the predetermined pattern regardless of a rotating position of the anvil.