A control system for a vehicular generator is provided which can be simplified in construction as the entire system to achieve cost reduction by using only a single control terminal that takes in a control signal from outside and by employing the control signal composed of only three kinds of signals. The system includes a generator 2 having an armature coil 202 and a field coil 201, a battery 4 to be charged by a power generation voltage, and a control circuit 1A that adjusts the power generation voltage by controlling to turn a field current supplied to the field coil 201 on and off in accordance with a battery voltage. A single control terminal C is provided on the control circuit 1A, and an external control unit 5 is connected to the control terminal C. The external control unit 5 supplies either one of a ground signal, an open signal and a pull-up signal to the control terminal C as a control signal in accordance with the battery voltage and an operating condition of the vehicle. The control circuit 1A interrupts the field current to stop power generation in response to the ground signal, adjust the power generation voltage to a first control voltage for normal time in response to the open signal, and adjusts the power generation voltage to a second control voltage higher than the first control voltage in response to the pull-up signal.