A hexagonal cell honeycomb structure body has cell walls arranged in a hexagonal shaped lattice, hexagonal shaped cells partitioned by the cell walls, and a skin layer with which the outside surface of the hexagonal cell honeycomb structure body is covered. An average thickness of basic cell walls is not more than 140 μm. A relationship of Dax/P≧0.13 is satisfied, where Dax is an average of diameters of inscribed circles, each of which is inscribed in a boundary part of three basic cell walls at a junction area between opening parts of adjacent three cells. On a cross sectional surface of the body, a surface of the basic cell wall has a concave part curved toward its inside direction, and an inside angle part of adjacent two basic cell walls has a curved shape, which smoothly connects the surfaces of the adjacent two basic cell walls.