First and second panel webs are superposed on each other and fed in their longitudinal direction. After a side gusset is temporarily fixed to the second panel web, the side gusset is divided by a dividing device to form a pair of open edges. The open edges are aligned with a line of weakness of the first panel web when the first and second panel webs are superposed on each other. The side gusset is temporarily sealed to the first and second panel webs by a temporary seal device to form temporary sealed sections. The first panel web is guided to be divided along its line of weakness and to be folded on the sides of its division edges. This guiding of the panel web and the temporary sealed sections cause the side gusset to be opened at the open edges to form open surfaces. An end face web is then disposed on the panel web to be superposed on folded parts of the panel web and the open surfaces of the side gusset.