A honeycomb structure body has a cylindrical outer peripheral wall, partition walls and cells. The cells are surrounded by the partition walls. In a radial cross section of the honeycomb structure body, the cells is divided into cell density sections having different cell densities formed from a central section to the outer peripheral wall. A boundary wall is formed between two cell density sections. Each cell density section has boundary cells and interior cells. The boundary cells are in contact with the boundary wall. The interior cells are not in contact with the boundary wall. In the radial cross section of the honeycomb structure body, an inscribed circle of each of the boundary cells has a diameter of not less than 0.5 mm. Further, an inscribed circle of an interior cell adjacent to the boundary cell also has a diameter of not less than 0.5 mm.