An exhaust gas purification system comprising an internal combustion engine for discharging an exhaust gas, a first exhaust gas purifying catalyst device for treating the exhaust gas, and a second exhaust gas purifying catalyst device for further treating the exhaust gas treated in the first exhaust gas purifying catalyst device, wherein; the first exhaust gas purifying catalyst device comprises a substrate, a lower catalyst layer disposed on the substrate, and an upper catalyst layer disposed on the lower catalyst layer and having a surface facing the flow path of the exhaust gas, the upper catalyst layer comprises a fine composite metal particle containing Pd and Rh, wherein the average ratio of the total number of Rh atoms to the total number of Pd and Rh atoms is 0.5 at% or more and 6.5 at% or less, and when diffraction angles 2θ indicative of the positions of diffraction peaks on the diffraction plane are specified by performing XRD analysis under the conditions that the X-ray wavelength is 1.5403 angstrom and the diffraction plane is the crystal lattice plane of Pd, the absolute value of the difference between the value of theoretical lattice constant B calculated from the following formula related to Vegard's law and the value of actual lattice constant C calculated from the following formula related to Bragg's law by using the values specified is 1.020×10 -3 (angstrom) or less: B = − 8.5459 × 10 − 2 × A + 3.890105 [wherein A is the average ratio of the total number of Rh atoms to the total number of Pd and Rh atoms]; C = λ × h 2 + k 2 + l 2 1 / 2 / 2 sin θ [wherein, λ is the X-ray wavelength, h, k and l are the Miller indices, and θ is a half of the diffraction angle 2θ]; and the upper catalyst layer contains the fine Pd-Rh composite metal particle in an amount of 0.1 g or more and 1.1 g or less per L of the volume of the substrate, and in the lower and upper catalyst layers, the position having a highest concentration of the fine Pd-Rh composite metal particle is the surface of the upper catalyst layer. the first exhaust gas purifying catalyst device comprises a substrate, a lower catalyst layer disposed on the substrate, and an upper catalyst layer disposed on the lower catalyst layer and having a surface facing the flow path of the exhaust gas, the upper catalyst layer comprises a fine composite metal particle containing Pd and Rh, wherein the average ratio of the total number of Rh atoms to the total number of Pd and Rh atoms is 0.5 at% or more and 6.5 at% or less, and when diffraction angles 2θ indicative of the positions of diffraction peaks on the diffraction plane are specified by performing XRD analysis under the conditions that the X-ray wavelength is 1.5403 angstrom and the diffraction plane is the crystal lattice plane of Pd, the absolute value of the difference between the value of theoretical lattice constant B calculated from the following formula related to Vegard's law and the value of actual lattice constant C calculated from the following formula related to Bragg's law by using the values specified is 1.020×10 -3 (angstrom) or less: B = − 8.5459 × 10 − 2 × A + 3.890105 [wherein A is the average ratio of the total number of Rh atoms to the total number of Pd and Rh atoms]; C = λ × h 2 + k 2 + l 2 1 / 2 / 2 sin θ [wherein, λ is the X-ray wavelength, h, k and l are the Miller indices, and θ is a half of the diffraction angle 2θ]; and the average ratio of the total number of Rh atoms to the total number of Pd and Rh atoms is 0.5 at% or more and 6.5 at% or less, and when diffraction angles 2θ indicative of the positions of diffraction peaks on the diffraction plane are specified by performing XRD analysis under the conditions that the X-ray wavelength is 1.5403 angstrom and the diffraction plane is the crystal lattice plane of Pd, the absolute value of the difference between the value of theoretical lattice constant B calculated from the following formula related to Vegard's law and the value of actual lattice constant C calculated from the following formula related to Bragg's law by using the values specified is 1.020×10 -3 (angstrom) or less: B = − 8.5459 × 10 − 2 × A + 3.890105 [wherein A is the average ratio of the total number of Rh atoms to the total number of Pd and Rh atoms]; C = λ × h 2 + k 2 + l 2 1 / 2 / 2 sin θ [wherein, λ is the X-ray wavelength, h, k and l are the Miller indices, and θ is a half of the diffraction angle 2θ]; and λ is the X-ray wavelength, h, k and l are the Miller indices, and θ is a half of the diffraction angle 2θ]; and the upper catalyst layer contains the fine Pd-Rh composite metal particle in an amount of 0.1 g or more and 1.1 g or less per L of the volume of the substrate, and in the lower and upper catalyst layers, the position having a highest concentration of the fine Pd-Rh composite metal particle is the surface of the upper catalyst layer.