PURPOSE: To expedite responsiveness of intake air amount correction when electric load rapidly increases at the time of idle driving and to restrict drop in engine rotation speed to maintain a stable rate of rotation speed by largely opening a bypass valve based on the electric load current value that is determined from a battery consumption current value and a power generation current value.

CONSTITUTION: Field current is turned ON/OFF according to battery voltage to control power generation level by an alternator 8, while the field current is gradually increased as electric load rapidly increases. The amount of intake air is increased/decreased by controlling the opening/closing of a bypass valve 5 for opening/closing an intake bypass route 4 at the time of idle driving so as to keep engine rotation speed at aimed level. The condition of a battery is detected, particularly, to estimate battery consumption current level, and an electric load current value is determined based on the sum of the estimated battery consumption current value and the alternator power generation current value. The bypass valve 5 is largely opened based on the electric load current value. High idle stability can thus be provided when the electric load is fluctuated.