To detect the degree of thermal deterioration of a storage and reduction type NOx catalyst.

This exhaust emission control device for an internal combustion engine having the storage and reduction type NOx catalyst 17 in an exhaust pipe 16 for the lean-combustible internal combustion engine comprises an incoming gas SOx sensor 23 provided at the upstream side of the NOx catalyst 17 and an outgoing gas SOx sensor 24 and an air/fuel ratio sensor 27 at the downstream side. SOx poisoning recovery treatment is executed to the NOx catalyst 17, and right after almost complete poisoning recovery, saturated NOx release treatment is executed to the NOx catalyst 17 and a change-to-rich time is measured and compared with a change-to-rich time for a new NOx catalyst 17 for judging the degree of thermal deterioration in accordance with the comparison value. Otherwise, the NOx clean-up rate of the NOx catalyst is measured for judging the degree of thermal deterioration. When the degree of thermal deterioration exceeds a preset level, thermal deterioration inhibiting means is operated for inhibiting further thermal deterioration.