Output waveform of the generator is improved through stabilization of field current by removing flywheel diode used to be required for automatic voltage regulator. Output electric current of excitation winding 3 is rectified by rectifier 8 and is supplied to field winding 5 of rotor 4. Impedance adjustment circuit 12 is provided to circuit where field current flows. Target electric current determination unit 10 determines target electric current (target field current) used to control output voltage of power generation winding 2 to the reference voltage. Impedance adjustment circuit 12 increases or decreases the impedance of field current circuit so that the field current detected by electric current detector 11 converges with target electric current.