A backflow preventing device comprising: a backflow preventing element connected between a power supply and a load and configured to prevent backflow of electric current from a side of the load toward a side of the power supply; a commutating device configured to perform a commutation operation for causing the electric current to flow to a commutation path connected to serve as a different path in parallel with the backflow preventing element; and a controller configured to cause the commutating device to perform the commutation operation, characterized in that: the commutating device comprises: a plurality of commutation paths connected in parallel, each commutation path comprising a commutation rectifying element (72a, 72b); a transformer comprising a single primary winding and a plurality of secondary windings, wherein a corresponding secondary winding is connected in series with the respective commutation rectifying element (72a, 72 b) of each commutation path; and a transformer driver having a commutation power supply and a commutation switch being connected to the primary winding of the transformer, wherein the controller is configured to open and close the commutation switch to control excitation current flowing toward the primary winding of the transformer from the commutation power supply, such that a voltage is applied to each secondary winding to perform the commutation operation. a backflow preventing element connected between a power supply and a load and configured to prevent backflow of electric current from a side of the load toward a side of the power supply; a commutating device configured to perform a commutation operation for causing the electric current to flow to a commutation path connected to serve as a different path in parallel with the backflow preventing element; and a controller configured to cause the commutating device to perform the commutation operation, characterized in that: the commutating device comprises: a plurality of commutation paths connected in parallel, each commutation path comprising a commutation rectifying element (72a, 72b); a transformer comprising a single primary winding and a plurality of secondary windings, wherein a corresponding secondary winding is connected in series with the respective commutation rectifying element (72a, 72 b) of each commutation path; and a transformer driver having a commutation power supply and a commutation switch being connected to the primary winding of the transformer, wherein the controller is configured to open and close the commutation switch to control excitation current flowing toward the primary winding of the transformer from the commutation power supply, such that a voltage is applied to each secondary winding to perform the commutation operation. a plurality of commutation paths connected in parallel, each commutation path comprising a commutation rectifying element (72a, 72b); a transformer comprising a single primary winding and a plurality of secondary windings, wherein a corresponding secondary winding is connected in series with the respective commutation rectifying element (72a, 72 b) of each commutation path; and a transformer driver having a commutation power supply and a commutation switch being connected to the primary winding of the transformer, wherein the controller is configured to open and close the commutation switch to control excitation current flowing toward the primary winding of the transformer from the commutation power supply, such that a voltage is applied to each secondary winding to perform the commutation operation.