A honeycomb segment has two kinds of cells having different cross-sectional areas in a cross-section cut along a plane perpendicular to the longitudinal direction of the segment. When the cells are cut along a plane perpendicular to the longitudinal direction of the cells, each cross-sectional area of first cells (large cells) is larger than that of second cells (small cells). The first cells are formed into an octagon where corners are cut off or into a square having circular arc corners. The second cells are formed into a square. The thickness of a 1-1 wall between the adjacent first cells is larger than that of a 1-2 wall between each first cell and each second cell adjacent to the first cell.