An engine comprising an exhaust passage having therein a NOx absorbent which absorbs the NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent is lean and releases the absorbed NOx when the air-fuel ratio of the exhaust gas flowing into the NOx absorbent becomes the stoichiometric air-fuel ratio or rich. When the air-fuel ratio of the air-fuel mixture should be changed over from lean to the stoichiometric air-fuel ratio, the air-fuel ratio of the air-fuel mixture is temporarily made rich and is then made the stoichiometric air-fuel ratio. <IMAGE>