A temperature sensor is disposed near an HC adsorbent in an exhaust system
of an internal combustion engine. After the internal combustion engine is
shut off, a deteriorated state of the HC adsorbent is evaluated based on a
value of the humidity detected by a humidity sensor. The deteriorated
state of the HC adsorbent is evaluated by comparing the detected value of
the humidity with a threshold established depending on the temperature
state near the HC adsorbent. In evaluating deteriorated state of the HC
adsorbent, a change in the output characteristics of the humidity sensor
depending on its temperature state is compensated for.
