Exhaust emission control device for internal combustion engine

1. Field of the Invention An object of the invention is to reliably judge if a NOabsorbent has deteriorated due to sulfur poisoning, or has heat deteriorated. At the time of executing rich spiking, a sulfur poisoning stoichiometric conversion time threshold value tm: LMT-S deterioration, for judging if a NOabsorbent has been sulfur poisoned, and a heat deterioration stoichiometric conversion time threshold value tm: LMT heat deterioration, for judging if the NOabsorbent has heat deteriorated to an extent that the NOabsorption capacity has been lost, are set in relation to the time (stoichiometric conversion time) taken for an output signal from a second Osensor to change from lean to rich. When a measurement value of the stoichiometric conversion time is shorter than the heat deterioration stoichiometric conversion time threshold value tm: LMT heat deterioration, it is judged that the NOabsorbent has completely heat deteriorated, and when this is longer than the heat deterioration stoichiometric conversion time threshold value tm: LMT heat deterioration, and shorter than the sulfur poisoning stoichiometric conversion time threshold value tm: LMT-S deterioration, it is judged that there is sulfur poisoning.