A controller of a generator (1) for a vehicle comprising a voltage regulator (3) for regulating a generated voltage to a predetermined voltage by performing on/off control of a field current through a switching element and performing variable control of a regulated voltage with a control signal from an external control unit (4) is further provided with a maximum duty factor limiting circuit (300) for limiting the maximum duty factor of the switching element (Q3) performing on/off control of the field current.