A voltage control device and a voltage control method capable of improving efficiency of a system as a whole even when fluctuation in voltage of a capacitor, which is a storage battery, is large are provided. A capacitor for supplying electric power to a predetermined load, an inverter connected to the load, an AC-coupling bi-directional DC-DC converter including two voltage source inverters of which direct-current terminals are connected in series so as to have additive polarity and a transformer for AC-coupling alternating-current terminals of the two voltage source inverters and having a predetermined leak inductance, with one of the two voltage source inverters connected in parallel to a capacitor, for outputting direct-current voltage obtained by increasing capacitor voltage of the capacitor to the inverter, and control unit for generating a command value of the direct-current voltage based on efficiency of the AC-coupling bi-directional DC-DC converter having a value corresponding to the capacitor voltage to output, are provided.