In an engine exhaust passage, in order from an exhaust upstream side, an NOx storage and reduction catalyst, NO oxidation catalyst, NO2 reduction catalyst, and selective reduction catalyst may be arranged. An air-fuel ratio of an exhaust gas which flows into the NOx storage and reduction catalyst may be temporarily switched to a rich air-fuel ratio which may be adapted for generating ammonia from NOx which may be stored in the NOx storage and reduction catalyst. The ammonia, which may be generated by the NOx storage and reduction catalyst, may be held at the selective reduction catalyst. The NOx which flows into the selective reduction catalyst may be reduced by the ammonia. The NO oxidation catalyst and NO2 reduction catalyst may be used to make an NO ratio of the exhaust gas which flows into the selective reduction catalyst approach an optimum ratio of the selective reduction catalyst.