An exhaust system comprising an exhaust gas path, and a honeycomb catalyst body disposed in the exhaust gas path, the honeycomb catalyst body comprising: a honeycomb structure having porous partition walls with which a plurality of cells extending from an inflow side end surface to an outflow side end surface are defined and formed to become through channels of an exhaust gas; outflow side plugged portions arranged in open frontal areas of the predetermined cells in the outflow side end surface, to form the inflow cells which are open in the inflow side end surface and are closed in the outflow side end surface; inflow side plugged portions arranged in open frontal areas of the remaining cells in the inflow side end surface, to form the outflow cells which are open in the outflow side end surface and are closed in the inflow side end surface; and porous catalyst layers formed on the surfaces of the partition walls on the side of the outflow cells, and having a catalyst to purify at least NOx included in the exhaust gas, wherein in the honeycomb structure, the inflow cells and the outflow cells are alternately arranged via the partition walls, and the plurality of cells are formed so that an open area of each of the outflow cells in a cross section of the outflow cell which is vertical to an extending direction of the cells is larger than an open area of each of the inflow cells in a cross section of the inflow cell which is vertical to the cell extending direction, a porosity of the partition walls of the honeycomb structure is from 25 to 55%, measured using a mercury porosimeter, a thickness of the partition walls is from 102 to 254μm, the catalyst layers are not formed on the surfaces of the partition walls on the side of the inflow cells, furthermore, either: (i) the catalyst layers are not formed on the surfaces of the pores in the partition walls, or (ii) the catalyst layers are formed on the surfaces of the pores in the partition walls, and a total cross sectional area of the catalyst layers formed on the surfaces of the pores in the partition walls is 5% or less of a total cross sectional area of the pores in the partition walls, this being calculated by taking ten images of size 500 μm x 500 μm of the partition walls and subjecting the images to image analysis to determine the cross sectional area of catalyst and the cross sectional area of pores, in a cross section parallel to the cell extending direction. a honeycomb structure having porous partition walls with which a plurality of cells extending from an inflow side end surface to an outflow side end surface are defined and formed to become through channels of an exhaust gas; outflow side plugged portions arranged in open frontal areas of the predetermined cells in the outflow side end surface, to form the inflow cells which are open in the inflow side end surface and are closed in the outflow side end surface; inflow side plugged portions arranged in open frontal areas of the remaining cells in the inflow side end surface, to form the outflow cells which are open in the outflow side end surface and are closed in the inflow side end surface; and porous catalyst layers formed on the surfaces of the partition walls on the side of the outflow cells, and having a catalyst to purify at least NOx included in the exhaust gas, wherein in the honeycomb structure, the inflow cells and the outflow cells are alternately arranged via the partition walls, and the plurality of cells are formed so that an open area of each of the outflow cells in a cross section of the outflow cell which is vertical to an extending direction of the cells is larger than an open area of each of the inflow cells in a cross section of the inflow cell which is vertical to the cell extending direction, a porosity of the partition walls of the honeycomb structure is from 25 to 55%, measured using a mercury porosimeter, a thickness of the partition walls is from 102 to 254μm, the catalyst layers are not formed on the surfaces of the partition walls on the side of the inflow cells, furthermore, either: (i) the catalyst layers are not formed on the surfaces of the pores in the partition walls, or (ii) the catalyst layers are formed on the surfaces of the pores in the partition walls, and a total cross sectional area of the catalyst layers formed on the surfaces of the pores in the partition walls is 5% or less of a total cross sectional area of the pores in the partition walls, this being calculated by taking ten images of size 500 μm x 500 μm of the partition walls and subjecting the images to image analysis to determine the cross sectional area of catalyst and the cross sectional area of pores, in a cross section parallel to the cell extending direction. (i) the catalyst layers are not formed on the surfaces of the pores in the partition walls, or (ii) the catalyst layers are formed on the surfaces of the pores in the partition walls, and a total cross sectional area of the catalyst layers formed on the surfaces of the pores in the partition walls is 5% or less of a total cross sectional area of the pores in the partition walls, this being calculated by taking ten images of size 500 μm x 500 μm of the partition walls and subjecting the images to image analysis to determine the cross sectional area of catalyst and the cross sectional area of pores, in a cross section parallel to the cell extending direction.