A honeycomb structural body having a plurality of cells and partition walls defining the cells, wherein a cross-section of each partition wall perpendicular to the flow direction of a fluid to be permeated has a grille shape of which gratings cross in x axis and Y axis directions, and there are at least two kinds of cells different in their cross-sectional areas perpendicular to the flow direction of a fluid to be filtered by virtue of changing intervals of the partition walls in the x axis direction and/or intervals of the partition walls in the y axis direction, as predetermined. Each cell is defined by two parallel partition walls having the same length and another two parallel partition walls having the same length crossing each other and being formed at the predetermined intervals. A die usable for forming such a honeycomb structure is also disclosed.