An apparatus for controlling an output from an AC generator for a vehicle is provided which enables wiring from an external control unit to a voltage regulator by using only one wiring line, and which is capable of adjusting an output voltage of the AC generator to a usual value in accordance with a duty ratio of a control signal supplied from the external control unit, and linearly controlling the generator output voltage to a desired value in relation to a change in the level of the control signal. The apparatus includes a control unit connected to a battery and a variety of sensors mounted on a vehicle for generating a control signal which has a duty ratio corresponding to a sensed operating condition of the vehicle; and a voltage regulator connected to the battery and the control unit for detecting an output voltage of the battery and controlling turning on and off of a field current supplied from the battery to a field coil of the AC generator in accordance with the detected battery voltage so as to adjust an output voltage of the AC generator to a level that is determined in accordance with the duty ratio of the control signal from the external control unit.