A throttle valve control system for an internal combustion engine mounted on a motor vehicle comprises a first device which determines necessity of controlling a throttle valve irrespective of operation of an accelerator pedal upon sensing a predetermined operation condition of the vehicle; a second device which devices a target opening degree of the throttle valve from the existing depression degree of an accelerator pedal with reference to a data-map, the data-map showing a relationship between the opening degree of the throttle valve, the depression degree of the accelerator pedal and the operation condition of the vehicle; a third device which compares the existing opening degree of the throttle valve and the derived target opening degree of the same; and a fourth device which controls the opening degree of the throttle valve in accordance with a result of the comparison of the opening degree.