An output voltage regulator for an engine-driven generator that can satisfactory suppress fluctuation in output voltage, even when an output waveform of the generator is distorted. An engine revolution period that represents a period of output voltage is detected by use of an ignition signal. Data that represents an output waveform is obtained by squaring and adding up an instantaneous value of the output voltage for one period of engine revolution. Furthermore, a square root of the data is extracted as an effective value. An operation duty of a transistor Q1 provided in an output circuit of an excitation winding L2 that supplies a field current "if" is determined based on a difference between the effective value and a target voltage. The field current "if" changes according to the difference, and the output voltage is thereby regulated according to a load.