A power conversion device provided between a DC power supply and an AC power system, and mutually converting AC and DC, the power conversion device comprising: respective phase arms in three phases each comprising a plurality of chopper cells connected in series; an upper arm and a lower arm formed by dividing the chopper cells in the phase arm into two; an output terminal provided between the upper arm and the lower arm, and configured to be connected to the AC power system; a first switch and a fourth switch provided at both ends of the phase arm, and provided between the phase arm and the DC power supply; a second switch and a third switch connected in series between the first switch and the fourth switch, and connected in parallel with the phase arm; respective neutral-points in the three phases provided between the second switch and the third switch, and connected with each other; and a controller configured to control the chopper cells and the first to fourth switches (41, 42, 43, 44), wherein the controller: is configured to control the first to fourth switches (41, 42, 43, 44) to form, in each voltage phase of the AC power system, combination circuits in which each of the phase arms in twophases are connected in parallel, and the phase arm in the remaining phase is connected in series to the phase arms connected in parallel; is configured to control the chopper cells to superimpose an AC current on a P-N circulating current that circulates between a positive side of the DC power supply and a negative side thereof via the combination circuit; is configured to control the AC current to suppress an imbalance in a voltage difference between the upper arm and the lower arm; characterized by that the controller is configured to superimpose two kinds of the AC current with different frequencies, and as for the imbalance that is the voltage difference between the upper arm and the lower arm, to suppress a normal-phase component and a reverse-phase component by the one AC current and to suppress a zero-phase component by the other AC current. respective phase arms in three phases each comprising a plurality of chopper cells connected in series; an upper arm and a lower arm formed by dividing the chopper cells in the phase arm into two; an output terminal provided between the upper arm and the lower arm, and configured to be connected to the AC power system; a first switch and a fourth switch provided at both ends of the phase arm, and provided between the phase arm and the DC power supply; a second switch and a third switch connected in series between the first switch and the fourth switch, and connected in parallel with the phase arm; respective neutral-points in the three phases provided between the second switch and the third switch, and connected with each other; and a controller configured to control the chopper cells and the first to fourth switches (41, 42, 43, 44), wherein the controller: is configured to control the first to fourth switches (41, 42, 43, 44) to form, in each voltage phase of the AC power system, combination circuits in which each of the phase arms in twophases are connected in parallel, and the phase arm in the remaining phase is connected in series to the phase arms connected in parallel; is configured to control the chopper cells to superimpose an AC current on a P-N circulating current that circulates between a positive side of the DC power supply and a negative side thereof via the combination circuit; is configured to control the AC current to suppress an imbalance in a voltage difference between the upper arm and the lower arm; is configured to control the first to fourth switches (41, 42, 43, 44) to form, in each voltage phase of the AC power system, combination circuits in which each of the phase arms in twophases are connected in parallel, and the phase arm in the remaining phase is connected in series to the phase arms connected in parallel; is configured to control the chopper cells to superimpose an AC current on a P-N circulating current that circulates between a positive side of the DC power supply and a negative side thereof via the combination circuit; is configured to control the AC current to suppress an imbalance in a voltage difference between the upper arm and the lower arm; characterized by that the controller is configured to superimpose two kinds of the AC current with different frequencies, and as for the imbalance that is the voltage difference between the upper arm and the lower arm, to suppress a normal-phase component and a reverse-phase component by the one AC current and to suppress a zero-phase component by the other AC current.