Abstract: A device for controlling output voltage of a converter is provided. The input voltage V.sub.R is detected by means of a voltage detecting device and the phase .PHI. of the input voltage is detected by means of a phase synchronizing circuit. The detected phase is input to an AC sine wave reference oscillating circuit in order to obtain an AC sine wave reference signal having the same phase as input voltage. The output of an amplitude instruction generating circuit is multiplied by an AC sine wave reference signal by means of a multiplier in order to obtain a converter current instruction I.sub.A *. The input current I.sub.R detected by means of a current detecting device is differentiated by means of an approximation differentiator and only the resonance frequency component is detected by means of a band-pass filter. The detected resonance frequency component is multiplied by a gain K by means of a proportion device and the output os subtracted from the converter current instruction I.sub.