PURPOSE: To prevent abnormal decrease and increase of idle engine speed by detecting atmospheric pressure, and by pressure-compensating composite control quantity of engine speed feedback compensation in response to difference between real and target engine speed, and of standard air control quantity.

CONSTITUTION: An air control valve 11 is provided with a bypass pipe 10 which bypasses a throttle valve 7 positioned at the downstream side of a fuel injection valve 5 inside an intake pipe 3. The air control valve 11 is duty-controlled by an electronic control unit 22 based on output signals of a pressure sensor 6, an idle switch 9, an ignition coil 12, an water temperature sensor 14 and the like. At the time of idle condition, opening of the air control valve 11 is controlled in response to composite control quantity of standard air control quantity and engine speed feedback compensation to eliminate the deference between target and real engine speed. In this case, atmospheric pressure is detected by the output of the pressure sensor 6 at the time of stopping an engine, and the composite control quantity is atmospheric pressure-compensated at the time of starting the engine.