| abstract |
(57) [Problem] To provide an exhaust gas purifying device capable of accurately determining the degree of deterioration even when a NOx purifying device is disposed downstream of a three-way catalyst. SOLUTION: An exhaust system of an internal combustion engine 1 includes a proportional oxygen concentration sensor 17, a three-way catalyst 14, a binary O2 sensor 18, The NOx purifying device 15 and the binary O2 sensor 19 are arranged in this order. When the output of the O2 sensor 18 changes when the air-fuel ratio is enriched, the O2 sensor 1 9, the timer measurement value tmMON2 up to the output change point is: Correction coefficient K set according to the degree of deterioration of three-way catalyst 14 It is corrected by MON2 (S66), and when the corrected timer measurement value tmMON2C is smaller than the reference time TNOXREF, it is determined that the NOx purification device 15 has deteriorated (S68, S69). |