The vehicle power-generation control unit has a first function enabled when a control signal received from outside designates a first mode to control an exciting current such that an output power of a vehicle generator is kept at a value specified by the control signal, a second function enabled when the control signal designates a second mode to control the exciting current such that a generation voltage of the vehicle generator is kept at a target voltage specified by the control signal, and a third function enabled when the control signal designates the first mode to monitor whether or not the generation voltage is in a predetermined voltage range and to disable the first function in order to control the exciting current such that the generation voltage is kept at a preset voltage upon detecting that the generation voltage is out of the predetermined voltage range.