A honeycomb structure includes multiple honeycomb segments joined by means of a bonding material. Each of the honeycomb segments has a plurality of passages which are defined by partition walls and extend in an axial direction, Flatness of outer walls to be joining planes of the honeycomb segments is 0.2 mm or more. The honeycomb structure is superior in durability, by which a honeycomb segment does not shift due to vibration or exhaust back pressure in use.