Abstract: A method and apparatus for detecting a functional condition of an NOx occlusion catalyst using an NOx sensor. In order to compensate for variations in oxygen concentration of the exhaust gas which would otherwise affect the detected NOx concentration downstream of the NOx occlusion catalyst, a relative value is calculated as the difference between the detected NOx concentration and the value of the detected NOx concentration initially after start of operation control of an internal combustion engine at a lean air-fuel ratio. The occlusion capability of the NOx occlusion catalyst is judged to have deteriorated when an increase in the relative value exceeds a predetermined value. On the other hand, the NOx occlusion catalyst is judged to have suffered an anomaly when the rate of increase in the relative value becomes greater than a predetermined allowable value.