A control device for a vehicle AC generator that feeds an in-vehicle battery and a vehicle load by rectifying an AC output of the vehicle AC generator, the control device comprising: a voltage control unit that repetitively gives a control cycle to a power switch element controlling an exciting current flowing through a field coil of the vehicle AC generator and controls an output voltage of the vehicle AC generator to take a target voltage value by adjusting an excitation duty indicating an ON time ratio of the power switch element in each control cycle, wherein the voltage control unit includes, target voltage adjusting means for adjusting the target voltage value in response to a target change command from an external control unit, and an excitation duty control circuit (60, 60A) that controls the excitation duty to increase according to an excitation duty control characteristic when the vehicle load increases and when the target voltage adjusting means reduces the target voltage value, the excitation duty control circuit (60, 60A) includes duty characteristic varying means for varying the excitation duty control characteristic, and the duty characteristic varying means varies a rate of increase of the excitation duty in a first region of the excitation duty control characteristic wherein the excitation duty is equal to or below a predetermined value, in comparison with a second region of the excitation duty control characteristic wherein the excitation duty is above the predetermined value, characterized in that the excitation duty control circuit (60, 60A) includes a load response control counter that counts a load response control clock and outputs a load response control count value, and the duty characteristic varying means determines the first region according to the load response control count value. a voltage control unit that repetitively gives a control cycle to a power switch element controlling an exciting current flowing through a field coil of the vehicle AC generator and controls an output voltage of the vehicle AC generator to take a target voltage value by adjusting an excitation duty indicating an ON time ratio of the power switch element in each control cycle, wherein the voltage control unit includes, target voltage adjusting means for adjusting the target voltage value in response to a target change command from an external control unit, and an excitation duty control circuit (60, 60A) that controls the excitation duty to increase according to an excitation duty control characteristic when the vehicle load increases and when the target voltage adjusting means reduces the target voltage value, target voltage adjusting means for adjusting the target voltage value in response to a target change command from an external control unit, and an excitation duty control circuit (60, 60A) that controls the excitation duty to increase according to an excitation duty control characteristic when the vehicle load increases and when the target voltage adjusting means reduces the target voltage value, the excitation duty control circuit (60, 60A) includes duty characteristic varying means for varying the excitation duty control characteristic, and the duty characteristic varying means varies a rate of increase of the excitation duty in a first region of the excitation duty control characteristic wherein the excitation duty is equal to or below a predetermined value, in comparison with a second region of the excitation duty control characteristic wherein the excitation duty is above the predetermined value, characterized in that the excitation duty control circuit (60, 60A) includes a load response control counter that counts a load response control clock and outputs a load response control count value, and the duty characteristic varying means determines the first region according to the load response control count value.