PURPOSE: To simplify constitution, improve the anti engine stall performance, and improve responsiveness and stability by increasing the control quantity of a flow rate control valve in a bypass side passage for idling by the value corresponding to the reduction state of the revolution speed of an internal combustion engine.

CONSTITUTION: The upstream side and downstream side of a throttle valve 5 are allowed to communicate through a bypass passage 35 for idling, and the revolution speed of an internal combustion engine 10 is kept at an aimed revolution speed by varying the control quantity of a flow rate control valve 36 installed in a side passage 35. When the reduction of the revolution speed is detected, the control quantity of the flow rate control valve 36 is increased by the value corresponding to the reduction state. At the time point when the revolution speed is increased by the increase of the control quantity, and the speed variation of the revolution speed becomes nearly zero, the aimed value of the parameter related to the torque of the internal combustion engine 10 is set, and the control quantity of the flow rate control valve 36 is reduced to a prescribed value permitting the maintenance of the aimed value.