Regular hexagonal cells at both an top surface and a bottom surface of a green honeycomb molded body are closed at the same time. Therefore, the regular hexagonal cells can be closed effectively. In addition, the regular hexagonal cells of the green honeycomb molded body are closed by welding partition walls with each other, by inserting each of closing protrusions of a closing jig into the plurality of regular hexagonal cells. Therefore, a closing paste like the conventional method is not required. In a case where a honeycomb structure is used for a diesel particulate filter, turbulence of exhaust gas flow, at the end surface of the side to supply the exhaust gas and/or the side to discharge the exhaust gas, is reduced, and pressure drop can be reduced.