An output voltage of an AC generator is produced through a full-wave rectifier and converted into a digital signal by an A/D converter. This digital signal is compared with a reference signal, and in accordance with the difference therebetween, the firing angle of a thyristor is controlled to regulate the field current. Synchronous point detectors detect a synchronous point of the output voltage of the AC generator. A microprocessor computes a firing angle of the thyristor in accordance with the difference between the digital signal and the reference signal and produces a signal representing the computed firing angle upon detection of the synchronous point. The thyristor is fired at the firing angle computed by the microprocessor.