A power conversion device comprising: arms corresponding to respective three phases, each of the arms comprising: a positive-side arm (U p, V p, W p) which comprises a plurality of converters connected in series and which has a first end connected to a positive-side DC terminal, each of the converters comprising a plurality of switching elements (21U, 21X) connected in series and a capacitor connected in parallel with the plurality of switching element connected in series; and a negative-side arm (U n, V n, W n) which comprises a plurality of converters connected in series and which has a first end connected to a negative-side DC terminal, each of the converters comprising a plurality of switching elements (21U, 21X) connected in series and a capacitor connected in parallel with the plurality of switching element connected in series; a converter transformer comprising a primary winding connected to three phase AC, and a secondary winding and a tertiary winding provided between a second end of the positive-side arm and a second end of the negative-side arm; wherein: the converter transformer comprises an iron core, the primary winding, and the secondary winding and the tertiary winding placed adjacent to the primary winding, the primary winding, the secondary winding, and the tertiary winding are placed concentrically in sequence relative to the iron core to equalize an impedance between a primary side and a secondary side, and an impedance between the primary side and a tertiary side, characterized in that the power conversion device comprises a tank which stores the converter transformer, and a gap is provided between the secondary winding and the tertiary winding, and the secondary winding and the tertiary winding are placed so that a DC leakage flux generated from the secondary winding passes through the iron core and the gap and a DC leakage flux generated from the secondary winding passes through the tank and the gap. arms corresponding to respective three phases, each of the arms comprising: a positive-side arm (U p, V p, W p) which comprises a plurality of converters connected in series and which has a first end connected to a positive-side DC terminal, each of the converters comprising a plurality of switching elements (21U, 21X) connected in series and a capacitor connected in parallel with the plurality of switching element connected in series; and a negative-side arm (U n, V n, W n) which comprises a plurality of converters connected in series and which has a first end connected to a negative-side DC terminal, each of the converters comprising a plurality of switching elements (21U, 21X) connected in series and a capacitor connected in parallel with the plurality of switching element connected in series; a positive-side arm (U p, V p, W p) which comprises a plurality of converters connected in series and which has a first end connected to a positive-side DC terminal, each of the converters comprising a plurality of switching elements (21U, 21X) connected in series and a capacitor connected in parallel with the plurality of switching element connected in series; and a negative-side arm (U n, V n, W n) which comprises a plurality of converters connected in series and which has a first end connected to a negative-side DC terminal, each of the converters comprising a plurality of switching elements (21U, 21X) connected in series and a capacitor connected in parallel with the plurality of switching element connected in series; a converter transformer comprising a primary winding connected to three phase AC, and a secondary winding and a tertiary winding provided between a second end of the positive-side arm and a second end of the negative-side arm; wherein: the converter transformer comprises an iron core, the primary winding, and the secondary winding and the tertiary winding placed adjacent to the primary winding, the primary winding, the secondary winding, and the tertiary winding are placed concentrically in sequence relative to the iron core to equalize an impedance between a primary side and a secondary side, and an impedance between the primary side and a tertiary side, characterized in that the power conversion device comprises a tank which stores the converter transformer, and a gap is provided between the secondary winding and the tertiary winding, and the secondary winding and the tertiary winding are placed so that a DC leakage flux generated from the secondary winding passes through the iron core and the gap and a DC leakage flux generated from the secondary winding passes through the tank and the gap. the converter transformer comprises an iron core, the primary winding, and the secondary winding and the tertiary winding placed adjacent to the primary winding, the primary winding, the secondary winding, and the tertiary winding are placed concentrically in sequence relative to the iron core to equalize an impedance between a primary side and a secondary side, and an impedance between the primary side and a tertiary side, characterized in that the power conversion device comprises a tank which stores the converter transformer, and a gap is provided between the secondary winding and the tertiary winding, and the secondary winding and the tertiary winding are placed so that a DC leakage flux generated from the secondary winding passes through the iron core and the gap and a DC leakage flux generated from the secondary winding passes through the tank and the gap.