The invention provides an electronic control unit for controlling an air/fuel ratio of an engine having a lean NOx catalyst in its exhaust system in order to prevent the decrease of the fuel efficiency without deterioration of the drivability due to a rich operation for reducing the NOx catalyst. The electronic control unit includes: an air/fuel ratio control system for changing the air/fuel ratio to a rich side when the amount of NOx occluded by the lean NOx catalyst exceeds a predetermined value and changing said air/fuel ratio back to a lean side when a first predetermined period has elapsed; a lagging system for lagging ignition timing in synchronization with the air/fuel ratio changing to the rich side by said air/fuel ratio control system; and decrementing system for decreasing the amount of the ignition timing lagged by the lagging system by a predetermined amount when a second predetermined period shorter than said first predetermined period has elapsed after the air/fuel ratio changing to the rich side. According to the invention, it is possible to prevent the decrease of the fuel efficiency without the deterioration of the drivability due to the rich operation to reduce the NOx catalyst because the ignition timing is lagged in synchronization with the air/fuel ratio changing to the rich side and the lagged amount is decremented after a predetermined period has elapsed. In one embodiment, the decrementing system gradually decrements said lagging amount up to a hold value.