A honeycomb structural body including a ceramic block having a flat shape and comprising a plurality of honeycomb units, a sealing material layer bonding the honeycomb units and comprising a sealing material paste, and a peripheral sealing material layer formed on a peripheral portion of the ceramic block. The flat shape has a cross-section which is bisected by an axis, the sealing material layer forms an angle diagonal with respect to the axis of the flat shape, the sealing material layer has end portions abutting against the peripheral sealing layer at a diagonal angle, and each of the honeycomb units has through holes extending in a longitudinal direction of the ceramic block and a partition wall portion between the through holes.