A vehicle AC generator control apparatus , for a vehicle AC generator wherein a rotor is coupled through a torque transfer member with an output axle of an internal combustion engine mounted in a vehicle, the control apparatus comprising a magnetic-field current control system that is configured to control a value of a magnetic-field current flowing in a magnetic-field winding of the vehicle AC generator , based on a magnetic-field current command value (b), wherein the magnetic-field current command value (b) is created based on an external command (a) from an external unit , characterized in that , the control apparatus further comprises an external command effective/ineffective determination unit that is configured to determine whether or not the external command (a) received from the external unit is effective and a voltage control system that can control an output voltage of the vehicle AC generator by controlling the magnetic-field current value (c) regardless of the external command (a), wherein in the case where the external command effective/ineffective determination unit determines that the external command (a) is effective, the magnetic-field control system controls the magnetic-field current value (c); and wherein in the case where the external command effective/ineffective determination unit determines that the external command is ineffective, the voltage control system controls the magnetic-field current value (c), and wherein the external command effective/ineffective determination unit is configured to determine that the external command is ineffective if a failure is anticipated. for a vehicle AC generator wherein a rotor is coupled through a torque transfer member with an output axle of an internal combustion engine mounted in a vehicle, the control apparatus comprising a magnetic-field current control system that is configured to control a value of a magnetic-field current flowing in a magnetic-field winding of the vehicle AC generator , based on a magnetic-field current command value (b), wherein the magnetic-field current command value (b) is created based on an external command (a) from an external unit , characterized in that , the control apparatus further comprises an external command effective/ineffective determination unit that is configured to determine whether or not the external command (a) received from the external unit is effective and a voltage control system that can control an output voltage of the vehicle AC generator by controlling the magnetic-field current value (c) regardless of the external command (a), wherein in the case where the external command effective/ineffective determination unit determines that the external command (a) is effective, the magnetic-field control system controls the magnetic-field current value (c); and wherein in the case where the external command effective/ineffective determination unit determines that the external command is ineffective, the voltage control system controls the magnetic-field current value (c), and wherein the external command effective/ineffective determination unit is configured to determine that the external command is ineffective if a failure is anticipated. <EOS>