Provided is a power conversion device, including a fault determination unit (11) for determining, based on phase voltages of a polyphase dynamo-electric machine (4) detected by phase voltage detection units (10), a power, earth, or open fault of armature windings of the polyphase dynamo-electric machine (4). The fault determination unit (11) determines, in a state that all power semiconductor switching elements (2) are in an off state and no induction voltage is generated in the armature windings of the polyphase dynamo-electric machine (4), the power fault when all the phase voltages are substantially equal to an anode potential of a DC power supply (3), the earth fault when all the phase voltages are substantially equal to a cathode potential of the DC power supply (3), and the open fault when all the phase voltages are not substantially the same potential.