A power conversion device which is provided between a DC terminal that includes a positive-side DC terminal and a negative-side DC terminal, and a three-phase AC terminal, and which is configured to mutually convert power between a DC system and a three-phase AC system, the power conversion device comprising: a positive-side arm and a negative-side arm each comprising a plurality of unit converters connected in multistage, each unit converter including a switching device (11a, 11b) and a capacitor, the positive-side arm and the negative-side arm being provided in each of the three phases, the positive-side arm and the negative-side arm being connected in series between the positive-side DC terminal and the negative-side DC terminal in each of the three phases; an AC circuit breaker provided in an AC current path between a three-phase AC system and a connection point between the positive-side arm and the negative-side arm; a short-circuit switch which is provided between the positive-side DC terminal and the negative-side DC terminal, and which is configured to connect the positive-side DC terminal and the negative-side DC terminal via a grounded point or directly; and a control device, wherein the control device comprises: a circuit-breaker control unit that is configured to output a breaking command to the AC circuit breaker; and a short-circuit-switch control unit that is configured to output a circuit close command to the short-circuit switch, wherein the control device is configured to cause the short-circuit switch control unit to output the close command to the short-circuit switch, characterised in that the control device comprises a detection unit that is configured to detect an accident occurrence at the connection point between the positive-side arm and the negative-side arm in a certain phase; in that a buffer reactor is provided at the positive-side arm and at the negative-side arm, respectively, the buffer reactor of the positive-side arm being connected in series between the AC terminal and the positive-side arm; and in that the control device is configured to cause the circuit-breaker control unit to output the breaking command to the AC circuit breaker after a signal indicating a close state of the short-circuit switch is received. a positive-side arm and a negative-side arm each comprising a plurality of unit converters connected in multistage, each unit converter including a switching device (11a, 11b) and a capacitor, the positive-side arm and the negative-side arm being provided in each of the three phases, the positive-side arm and the negative-side arm being connected in series between the positive-side DC terminal and the negative-side DC terminal in each of the three phases; an AC circuit breaker provided in an AC current path between a three-phase AC system and a connection point between the positive-side arm and the negative-side arm; a short-circuit switch which is provided between the positive-side DC terminal and the negative-side DC terminal, and which is configured to connect the positive-side DC terminal and the negative-side DC terminal via a grounded point or directly; and a control device, wherein the control device comprises: a circuit-breaker control unit that is configured to output a breaking command to the AC circuit breaker; and a short-circuit-switch control unit that is configured to output a circuit close command to the short-circuit switch, wherein the control device is configured to cause the short-circuit switch control unit to output the close command to the short-circuit switch, characterised in that the control device comprises a detection unit that is configured to detect an accident occurrence at the connection point between the positive-side arm and the negative-side arm in a certain phase; in that a buffer reactor is provided at the positive-side arm and at the negative-side arm, respectively, the buffer reactor of the positive-side arm being connected in series between the AC terminal and the positive-side arm; and in that the control device is configured to cause the circuit-breaker control unit to output the breaking command to the AC circuit breaker after a signal indicating a close state of the short-circuit switch is received. a circuit-breaker control unit that is configured to output a breaking command to the AC circuit breaker; and a short-circuit-switch control unit that is configured to output a circuit close command to the short-circuit switch, wherein the control device is configured to cause the short-circuit switch control unit to output the close command to the short-circuit switch, characterised in that the control device comprises a detection unit that is configured to detect an accident occurrence at the connection point between the positive-side arm and the negative-side arm in a certain phase; in that a buffer reactor is provided at the positive-side arm and at the negative-side arm, respectively, the buffer reactor of the positive-side arm being connected in series between the AC terminal and the positive-side arm; and in that the control device is configured to cause the circuit-breaker control unit to output the breaking command to the AC circuit breaker after a signal indicating a close state of the short-circuit switch is received.