When a fluctuation of engine speed is detected during idling of an engine, an opening degree of the flow control valve provided in an idle bypass pipe is changed at a comparatively high rate of change. Based upon this, when the rate of change of the engine speed approaches zero, it rapidly changes the opening degree to maintain a value related to intake pipe pressure or an accumulated value of the intake pipe pressure and the engine speed at that point, or until the value is reached at a range which a gentle change is possible. Consequently, for example, when the engine speed drops, the intake air flow is increased at a comparatively high rate of change, and it is possible to prevent engine stalling. Further more, when the rate of change of the engine speed passes through zero and begins to rise, the intake air flow is rapidly reduced until the value is reached the range at which the torque at that point can be maintained, and so called quick response due to overcontrol is prevented. In this way, with a high control gain, it performs idle speed control having more favorable stability.