In a lean-burn engine equipped, in the exhaust pipe, with NOx trap catalyst that collects NOx by absorption or occlusion, the rich spike start timing and rich spike volume are optimized.The above subject is achieved by an engine control system equipped, in the downstream side of the NOx trap catalyst, with a NOx sensor that detects the NOx component in the exhaust, NOx trap catalyst model, and a device that controls the engine operating condition based on the outputs of the NOx trap catalyst model and NOx sensor.