Throttle control system for a vehicular internal combustion engine equipped with a stepper motor connected to a throttle valve provided at an engine air intake passage, wherein the position (the opening degree) of the throttle valve is controlled by the stepper motor in response to the vehicular engine operation. Various throttle positions are once determined including positions for engine idling (S18) and vehicle cruising (S24) and the largest of the positions is selected (S28) for optinally satisfying the whole operating condition. In a first embodiment, when the engine is idling, the throttle valve is controlled such that engine speed converges toward a target speed. In another embodiment, the stepper motor is varied its pulse rate and chopping duty for optimal control of the throttle position according to the vehicular engine operation.