A honeycomb structure having multiple honeycomb units united through a seal material layer, wherein each honeycomb unit has multiple through holes arranged side by side in a longitudinal direction and separated from each other by the wall surfaces of the through holes, is disclosed. The honeycomb units include at least: ceramic particles; and at least one of inorganic fibers and whiskers. At least one of the honeycomb units has a cross section perpendicular to a longitudinal direction thereof, the cross section having an area greater than or equal to about 5 cm2 and less than or equal to about 50 cm2. The flatness of the exterior wall of each of the honeycomb units is about 0.1 mm to about 1.5 mm.