A honeycomb structural body has cells surrounded by cell walls arranged in a lattice-like shape. A cross section is divided into a central section and an outer peripheral section. The outer peripheral section surrounds the central section through a boundary partition wall. The outer peripheral section is lower in a cell density than the central section. The cells contain imperfectly shaped cells in contact directly with the boundary partition wall. In the imperfectly shaped cells, first specific imperfectly shaped cells are smaller in opening area than second specific imperfectly shaped cells. Each first specific imperfectly shaped cells has an opening area of not more than a predetermined area S=(P−T)2/8. P and T indicate a cell pitch and a thickness of each cell wall, respectively. The opening section of the first specific imperfectly shaped cells is closed by the same raw material of the cell walls.