PURPOSE: To clear the regulation value of NOx over the whole area of a load, and to prevent the excess of the speed fluctuation quantity over the allowable limit by deciding the allowable limit of the rotation fluctuation quantity of an engine corresponding to a detected load and the air ratio at the time when the rotating fluctuation quantity achieves the allowable limit.

CONSTITUTION: In a lean combustion engine 1, a load sensor 12 as a load detecting means and a rotation pick-up (rotation sensor) 11 are provided, and they are connected to a control unit 10. This unit 10 is provided with a central processing device 20, a means 21 for deciding allowable limit of fluctuation quantity of the engine speed, an air ratio deciding means 22 for deciding the air ratio so that the speed fluctuation quantity achieves the allowable limit. At the time of operation, the unit 10 detects a load with the load sensor 12, and decides the allowable limit of the fluctuation quantity of the engine speed with the rotation fluctuation allowable limit deciding means 21 on the basis of the detected load. The unit 10 decides the air ratio with the air ratio deciding means 22 on the basis of the allowable limit so that the rotation fluctuation quantity achieves the allowable limit.