A system for controlling a vehicular generator, includes: a voltage controlling device 1 for adjusting a generator output voltage of a generator 10 connected to an in-vehicle battery 4 to a predetermined voltage, the voltage controlling device 1 including an external voltage sensing terminal S for detecting an external voltage of the generator 10, and the generator output voltage being adjusted through on/off control of a field current of the generator 10 in accordance with the external voltage, in which an external control unit 5 is inserted between the external voltage sensing terminal S and the battery 4 to cause a voltage drop. Consequently, it is possible to dispense with an exclusive input terminal for external signal, thereby enabling simple system configuration cost reduction.