PURPOSE: To reduce the fluctuation of the revolving number of engine due to the loading-ON or OFF of a load and improve the efficiency of the engine by a method wherein the operations of actual loading-ON or OFF are delayed for a given time when the set condition of the loading-ON or OFF of the load is changed.

CONSTITUTION: When the set condition of the load 110 is shifted from loading-ON to OFF, the opening degree (d) of a throttle valve is increased and the revolving number is increased by the adding operation of the compensation amount of the load. On the other hand, a load driving device 100 outputs a signal (e) to apply the load 110 ON after a given time ΔT1 from the detection of the shift of the load at a time t2. The opening degree (d) of the valve is changed to the object opening degree at the time t1 when the load 110 is loaded and the revolving number is also increased, therefore, the revolving number with never be dropped due to the change of the load. When the load is shifted from ON to OFF, the throttle valve 11 is closed and the opening degree (d) is decreased by the adding operation of the compensating amount of the load. Subsequently, the load driving device 100 cuts OFF the load after a given time ΔT2 from the detection of the change from ON to OFF, thereby reducing the fluctation of the revolving number due to the cutting-OFF of the load.