A honeycomb structure includes a ceramic block including honeycomb fired bodies. The honeycomb fired bodies include an assembly including a plurality of substantially quadrangular-cross-section units which are combined with one another with an adhesive layer interposed between the plurality of substantially quadrangular-cross-section units. A peripheral face of the assembly of substantially quadrangular-cross-section units has a concave portion and a convex portion formed in a substantially step-shaped pattern. At least one substantially triangular-cross-section unit has an outer wall on a periphery portion of the at least one substantially triangular-cross-section unit. The at least one substantially triangular-cross-section unit is fit in the concave portion with the adhesive layer interposed between the at least one substantially triangular-cross-section unit and the concave portion. A sealing material layer is provided on a peripheral face of the ceramic block. The sealing material layer has partially different thickness.