A power conversion device, in which a plurality of phase bridge circuits each including power semiconductor switching elements, which are connected in series with each other to construct an upper arm and a lower arm, are connected in parallel to one another, both terminals of the plurality of phase bridge circuits are connected to a DC power supply, which is chargeable and dischargeable, and nodes between the power semiconductor switching elements of the upper arm and the lower arm of the plurality of phase bridge circuits are connected to AC terminals of armature windings of a polyphase dynamo-electric machine, arranged to perform AC-DC power conversion or DC-AC power conversion, the power conversion device comprising: an internal power supply circuit; a driving circuit arranged to turn on or off the power semiconductor switching elements; a discharge type constant current circuit arranged to connect an output terminal of the internal power supply circuit and one of the AC terminals of the armature windings of the polyphase dynamo-electric machine, to thereby cause a constant current to flow from the output terminal of the internal power supply circuit to the one of the AC terminals of the armature windings of the polyphase dynamo-electric machine; a backflow prevention diode connected in series with the discharge type constant current circuit, arranged to prevent a backflow current from the one of the AC terminals of the armature windings of the polyphase dynamo-electric machine to the internal power supply circuit; a pull-down resistor arranged to connect others of the AC terminals of the armature windings of the polyphase dynamo-electric machine, to which the discharge type constant current circuit is not connected, to a cathode potential of the DC power supply; a phase voltage detection unit arranged to detect phase voltages of the armature windings of the polyphase dynamo-electric machine; and a fault determination unit arranged to determine a power fault, an earth fault, and an open fault of the armature windings of the polyphase dynamo-electric machine based on the phase voltages of respective phases which are detected by the phase voltage detection unit, wherein the fault determination unit is arranged to determine, in a state in which all the power semiconductor switching elements are in an off state and no induction voltage is generated in the armature windings of the polyphase dynamo-electric machine, the power fault when all the phase voltages are substantially equal to an anode potential of the DC power supply, the earth fault when all the phase voltages are substantially equal to the cathode potential of the DC power supply, and the open fault when all the phase voltages are not substantially the same potential. an internal power supply circuit; a driving circuit arranged to turn on or off the power semiconductor switching elements; a discharge type constant current circuit arranged to connect an output terminal of the internal power supply circuit and one of the AC terminals of the armature windings of the polyphase dynamo-electric machine, to thereby cause a constant current to flow from the output terminal of the internal power supply circuit to the one of the AC terminals of the armature windings of the polyphase dynamo-electric machine; a backflow prevention diode connected in series with the discharge type constant current circuit, arranged to prevent a backflow current from the one of the AC terminals of the armature windings of the polyphase dynamo-electric machine to the internal power supply circuit; a pull-down resistor arranged to connect others of the AC terminals of the armature windings of the polyphase dynamo-electric machine, to which the discharge type constant current circuit is not connected, to a cathode potential of the DC power supply; a phase voltage detection unit arranged to detect phase voltages of the armature windings of the polyphase dynamo-electric machine; and a fault determination unit arranged to determine a power fault, an earth fault, and an open fault of the armature windings of the polyphase dynamo-electric machine based on the phase voltages of respective phases which are detected by the phase voltage detection unit, wherein the fault determination unit is arranged to determine, in a state in which all the power semiconductor switching elements are in an off state and no induction voltage is generated in the armature windings of the polyphase dynamo-electric machine, the power fault when all the phase voltages are substantially equal to an anode potential of the DC power supply, the earth fault when all the phase voltages are substantially equal to the cathode potential of the DC power supply, and the open fault when all the phase voltages are not substantially the same potential.