An exhaust gas purification device that is arranged in an exhaust path of an internal combustion engine and purifies exhaust gas discharged from the internal combustion engine, the exhaust gas purification device comprising: a wall-flow-structured base material having input-side cells in each of which only an end on an exhaust-gas inflow side is opened, output-side cells which are arranged adjacent to the input-side cells and in each of which only an end on an exhaust-gas outflow side is opened, and a porous partition wall that partitions the input-side cells and the output-side cells; and a catalyst layer formed inside the partition wall by the use of a catalyst layer forming slurry having a viscosity η 400 when a shear speed is 400 s -1 of 50 mPas or less, and the average particle size of the particles in the slurry being about 1/50 to 1/3 of the average pore size of the partition wall, wherein the catalyst layer is formed in a region that extends from a surface of the partition wall contacting the input-side cells or the output-side cells and that covers at least 90% of a thickness of the partition wall, the catalyst layer being held by surfaces of inside pores of the partition wall in the region, and a relationship among an average filling ratio A of the catalyst layer held by pores having a pore size of 5 μm or more and less than 10 μm, an average filling ratio B of the catalyst layer held by pores having a pore size of 10 μm or more and less than 20 μm, and an average filling ratio C of the catalyst layer held by pores having a pore size of 20 μm or more and less than 30 μm among the inside pores of the partition wall in which the catalyst layer is held, satisfies a following expression A < B < C, the average filling ratio as measured according to the method defined in the description, and pressure losses P 10, Pso and P 90 which are obtained by cutting out a part corresponding to 10%, 50%, and 90% of a length of the base material from the end on the exhaust gas inflow side to a downstream side of the base material and causing gas to flow through the part satisfy the expressions 0.9 ≤ P 10 /P 50 ≤ 1.1 and 0.9 ≤ P 90 /P 50 ≤ 1.1. a wall-flow-structured base material having input-side cells in each of which only an end on an exhaust-gas inflow side is opened, output-side cells which are arranged adjacent to the input-side cells and in each of which only an end on an exhaust-gas outflow side is opened, and a porous partition wall that partitions the input-side cells and the output-side cells; and a catalyst layer formed inside the partition wall by the use of a catalyst layer forming slurry having a viscosity η 400 when a shear speed is 400 s -1 of 50 mPas or less, and the average particle size of the particles in the slurry being about 1/50 to 1/3 of the average pore size of the partition wall, wherein the catalyst layer is formed in a region that extends from a surface of the partition wall contacting the input-side cells or the output-side cells and that covers at least 90% of a thickness of the partition wall, the catalyst layer being held by surfaces of inside pores of the partition wall in the region, and a relationship among an average filling ratio A of the catalyst layer held by pores having a pore size of 5 μm or more and less than 10 μm, an average filling ratio B of the catalyst layer held by pores having a pore size of 10 μm or more and less than 20 μm, and an average filling ratio C of the catalyst layer held by pores having a pore size of 20 μm or more and less than 30 μm among the inside pores of the partition wall in which the catalyst layer is held, satisfies a following expression A < B < C, the average filling ratio as measured according to the method defined in the description, and pressure losses P 10, Pso and P 90 which are obtained by cutting out a part corresponding to 10%, 50%, and 90% of a length of the base material from the end on the exhaust gas inflow side to a downstream side of the base material and causing gas to flow through the part satisfy the expressions 0.9 ≤ P 10 /P 50 ≤ 1.1 and 0.9 ≤ P 90 /P 50 ≤ 1.1.