The invention provides a control device of a frequency converter, a control method of a frequency converter, and a control device of a variable speed pumped storage power generation system that can maintain a voltage balance using a high-efficiency and small-capacity energy storage element even in the case of a low output AC frequency. In the control device of a frequency converter, a rotating machine including a synchronous machine is interconnected to a power system. The frequency converter includes a plurality of sets of arms in which a plurality of unit converters are connected in series, capacitors are connected in parallel in the plurality of unit converters, one ends of the series connected unit converters are connected to respective phases of an alternating current, and the other ends of the series connected unit converters are connected to terminals of a direct current. The control device includes an AC voltage removal calculation unit configured to detect at least voltages of the capacitors in the plurality of unit converters that form the arms, extract a DC component from the capacitor voltages, and input the DC component; an arm voltage command generation unit configured to generate an arm voltage command value; a carrier signal generation unit configured to generate a carrier signal; and a gate pulse generation unit that is provided with a modulated wave generator configured to correct the arm voltage command value to be supplied by the arm voltage command generation unit and that uses the corrected arm voltage command value and the carrier signal to determine a gate command for each of the plurality of unit converters in respective arms. The modulated wave generator uses the DC component calculated in the AC voltage removal calculation unit to correct the arm voltage command value so that magnitude of the voltages of the capacitors in the unit converters of the arms does not fluctuate.