PURPOSE: To prevent degradation of drivability caused by speed change operation of an automatic transmission conducted following the switching of an air fuel ratio to a rich level so as to regenerate a NOX absorbent.

CONSTITUTION: A NOX absorbent 18 is arranged on the exhaust passage 17 of an engine 1, and normally the engine is driven at a lean air fuel ratio so as to absorb NOX contained in exhaust into the NOX absorbent, while an engine air fuel ratio is switched into a rich air fuel ratio when the amount of NOX absorbed is increased, so as to regenerate the NOX absorbent. An electronic control circuit 30 for controlling the engine is provided and speed change operation is conducted by an automatic transmission 40 based on the throttle opening and vehicle running speed, while it is estimated from the rate of change in the throttle opening and the vehicle running speed, whether speed change operation is conducted in the future within a predetermined period of time, and when speed change operation is estimated, switching to the rich air fuel ratio so as to regenerate NOX absorbent is inhibited for a period from estimation to a point where speed change operation is actually conducted.