An NOx sensor 27 detects the concentration of NOx released from an occlusion type NOx catalyst 25 into the atmosphere. Based on the output of the NOx sensor 27, a total NOx discharge A is computed. When the total NOx discharge A reaches an NOx emission judgment amount A0, corresponding to an NOx emission judgment amount designated by NOx emission regulations, before a distance traveled C reaches a predetermined distance traveled C0, an exhaust air-fuel ratio is changed to a rich air-fuel ratio to release NOx from the occlusion type NOx catalyst 25 efficiently. Then, the exhaust air-fuel ratio is changed to a stoichiometric air-fuel ratio to purify and reduce NOx by a three-way catalyst function.