A honeycomb structural body has plural cell density sections having a cell density which is changed stepwise in a radial direction. A partition wall is formed between adjacent cell density sections. The cell density sections have a high cell density section having a maximum cell density, excepting an outermost cell density section formed at an outermost side, and a low cell density section having a minimum cell density, excepting an innermost cell density section formed at an innermost side. A relationship of V−Va≧Vb+Vs is satisfied, where V indicates a volume of the honeycomb structural body if the overall honeycomb structural body is composed of the high cell density section, Va indicates a volume of the high cell density section, Vb indicates a volume of the cell density section, and Vs indicates a volume of the boundary wall which separates the low cell density section from the cell density section formed immediately inside of the low cell density section.