A honeycomb structural body has a central part, an outer-circumferential part and a boundary-partition wall. The central part has central basic-cell walls and central reinforced-cell walls. The outer-circumferential part has outer-circumferential basic-cell walls and outer-circumferential reinforced-cell walls. The body satisfies R1/P1≧0.5, R2/P2≧0.5, T10≧T20, T11>T21, T3>T10, T3>T20, T3≧T11 and T3>T21, where R1 indicates a distance from the boundary-partition wall toward a radially-inward direction of the body, P1 indicates an average cell-pitch in the central part, R2 indicates a distance from the boundary-partition wall toward a radially outward direction of the body, P2 indicates an average cell-pitch in the outer-circumferential part, T10 indicates an average-thickness of the central basic-cell walls, T20 indicates an average-thickness of the outer-circumferential basic-cell walls, T11 indicates an average-thickness of the central reinforced-cell walls, T21 indicates an average-thickness of the outer-circumferential reinforced-cell walls, and T3 indicates an average-thickness of the boundary-partition wall.