When an operation causing a decrease of the engine load is requested, for example, by switching off an air conditioner switch, an idle speed control system calculates a fuel decrease quantity from an engine load decrease quantity, and further calculates an air decrease quantity in accordance with a desired air fuel ratio after the engine load decrease, and the fuel decrease quantity. The control system decreases the air supply quantity to the engine by the air decrease quantity. Then, after the elapse of a preset time interval, the system controls the external load such as the air conditioner so as to reduce the engine load (for example, by turning off the air conditioner), and decrease the fuel injection quantity by the fuel decrease quantity. When the air conditioner is switched on, for example, the control system calculates a fuel increase quantity from the engine load increase, and further calculates an air increase quantity in accordance with the fuel increase quantity and a desired air fuel ratio after the engine load increase. Then, the control system controls the external load to increase the engine load, increases the fuel injection quantity and increases the air supply quantity.