When the engine RPM falls below a lower limit of a target RPM of idling while an engine is in an idle operation, a control valve is shifted to the open-loop control mode instead of the conventional feedback control mode so as to enable the engine RPM range to be quickly increased up to within the target RPM of idling rotations. Said control valve is disposed in a bypass which communicates with the upstream side and the downstream side of the throttle valve in intake passage to adjust the volume of an inspired air. The solenoid current command (amount of control) for adjusting the opening degree of the control valve during the period of the open-loop control mode is set at a level higher than that immediately before the feedback control mode shifts to the open-loop control mode.