After the start of operation of an engine (1), the characteristic change of a humidity sensor (19) is detected on the basis of the transition characteristic of output date from the humidity sensor (19) with respect to integrated water quantity data indicating the integrated water quantity fed to an HC adsorbent (7) through exhaust gases. Further, the data concerning the characteristics of the individual humidity sensors (19) are held by the resistance values of resistor elements (21) installed in the connectors or the like of the humidity sensors (19). Parameters for grasping the state of the HC adsorbent (7) are corrected according to the detected characteristic change of the humidity sensor (19) and the characteristics of the other individual humidity sensors (19), and the state of the HC adsorbent is grasped on the basis of the corrected parameters. This compensates the influence of the characteristic change of the humidity sensor and the influence of variation of the characteristic of each individual humidity sensor, thereby properly monitoring the state of degradation or the like of the hydrocarbon adsorbent (HC adsorbent).
