An idle rotational speed controller of an internal combustion engine includes an intake path for supplying air to a combustion chamber. An auxiliary intake path communicates the intake path on upstream and downstream sides of a throttle valve in the intake path. An opening/closing type control valve in the auxiliary intake path controls an idling intake amount. An intake pressure detecting unit detects an intake pressure of the intake path, and a fuel supplying unit controls the amount of fuel supplied to the combustion chamber based on the detected intake pressure. A controlling unit synchronizes a drive reference position for driving to open or close the control valve with a timing of detecting the intake pressure by the intake pressure detecting unit.