The present invention includes recognizing, as positions of outermost circumferential cells, positions each of which is calculated from an average cell pitch of usual cells and each of which is present on an inner side from a circumference of an end face of a honeycomb structure; disposing, on the basis of the recognition, virtual perforation regions at positions on a sheet which correspond to the outermost circumferential cells to be plugged, each of the virtual perforation regions being divided into a plurality of squares; and performing perforation processing of perforating at least one of a plurality of squares in each of the virtual perforation regions. Positions and a number of squares to be perforated in each of the virtual perforation regions are individually set in accordance with each of the positions of the outermost circumferential cells to be plugged to which each of the virtual perforation regions corresponds.