The appts. comprises an idle speed control device for controlling a throttle valve on the basis of a throttle opening calculated so as to make an actual speed of the internal combustion engine in idle operation equal to a target speed stored beforehand for idle operation. A comparison device compares the throttle opening in idle operation calculated by the idle speed control device with an upper limit value and a lower limit value preset for the throttle opening in idle operation. A correction device corrects a full closing reference position of the throttle opening on the basis of the comparison result of the comparison device. The correction device comprises a device for increasing the full closing reference position by a predetermined value when the throttle opening in idle operation is greater than or equal to the upper limit value and a device for decreasing the full closing reference position by a predetermined value when the throttle opening in idle operation is less than or equal to the lower limit value.
