A sealed honeycomb structure comprising: a honeycomb structure having porous walls dividedly forming a plurality of cells extending from an end surface of an inlet side to an end surface of an outlet side, the plurality of cells being a plurality of fluid channels; an outlet side sealing portion mounted at openings of predetermined cells in the end surface of the outlet side so as to form inlet cells where the end surface of the inlet side is open and the end surface of the outlet side is sealed; and an inlet side sealing portion mounted at openings of remaining cells in the end surface of the inlet side so as to form outlet cells where the end surface of the outlet side is open and the end surface of the inlet side is sealed, wherein the inlet cells and the outlet cells are disposed alternately across the walls, and an opening area of the inlet cell on a cross-section vertical to an extending direction of the cell is larger than that of the outlet cell on the cross-section vertical to the extending direction of the cell, wherein at least one cell among the outlet cells is a reinforced cell where a reinforcing part for reinforcing the outlet cell is formed at at least one corner portion at which the walls on the cross-section vertical to the extending direction of the cell cross each other, characterized in that the inlet cell is provided with a reinforcing part for reinforcing the inlet cell at at least one corner portion at which the walls on the cross-section vertical to the extending direction of the cell cross each other, and wherein the number of the corner portions reinforcing the inlet cell is smaller than the number of the corner portions reinforcing the outlet cell, wherein the reinforced cell comprises a reinforced corner portion at which the reinforcing part is formed and a non-reinforced corner portion at which the reinforcing part is not formed. a honeycomb structure having porous walls dividedly forming a plurality of cells extending from an end surface of an inlet side to an end surface of an outlet side, the plurality of cells being a plurality of fluid channels; an outlet side sealing portion mounted at openings of predetermined cells in the end surface of the outlet side so as to form inlet cells where the end surface of the inlet side is open and the end surface of the outlet side is sealed; and an inlet side sealing portion mounted at openings of remaining cells in the end surface of the inlet side so as to form outlet cells where the end surface of the outlet side is open and the end surface of the inlet side is sealed, wherein the inlet cells and the outlet cells are disposed alternately across the walls, and an opening area of the inlet cell on a cross-section vertical to an extending direction of the cell is larger than that of the outlet cell on the cross-section vertical to the extending direction of the cell, wherein at least one cell among the outlet cells is a reinforced cell where a reinforcing part for reinforcing the outlet cell is formed at at least one corner portion at which the walls on the cross-section vertical to the extending direction of the cell cross each other, characterized in that the inlet cell is provided with a reinforcing part for reinforcing the inlet cell at at least one corner portion at which the walls on the cross-section vertical to the extending direction of the cell cross each other, and wherein the number of the corner portions reinforcing the inlet cell is smaller than the number of the corner portions reinforcing the outlet cell, wherein the reinforced cell comprises a reinforced corner portion at which the reinforcing part is formed and a non-reinforced corner portion at which the reinforcing part is not formed.