EXHAUST-GAS PURIFICATION DEVICE FOR INTERNAL COMBUSTION ENGINE

A proportional oxygen sensor, a three-way catalyst, a binary O2 sensor, a NOx purification device, and a binary O2 sensor are positioned in the named order in the exhaust system of an internal combustion engine. A correction coefficient KMON2, which is set in accordance with the degree of deterioration of a three-way catalyst, is used to correct the timer value tmMON2 used to measure the time extending from the time at which the output of the O2 sensor is changed when the air-fuel ratio is increased, until the time at which the output of the O2 sensor is changed. When the corrected timer value tmMON2 is smaller than a reference time TNOxREF, it is ascertained that deterioration of the NOx purification device has occurred.