A power conversion device comprising: first and second DC connectors (3a, 3b) supplying and receiving DC power; an AC connector (3c-3e) supplying and receiving AC power; a first arm (A1-A3) connected between the first DC connector and the AC connector; and a second arm (A4-A6) connected between the AC connector and the second DC connector, each of the first and second arms including a plurality of cascaded unit cells (10, 10A, 10B), each unit cell having a switching element (S1, S2/S1-S4) and a capacitor, and at least some of the plurality of unit cells in at least one of the first and second arms being helically cascaded, wherein at least one of the first and second arms (A1-A3, A4-A6) includes first to Qth racks (LK1, LK2), each rack has first to Nth stages (ST1-ST5, ST11-ST15) sequentially disposed in a direction of height, M cascaded unit cells (10, 10A, 10B) are mounted on each stage, such that for each integer n not smaller than 1, and not larger than N, QxM unit cells are mounted on the nth stages of the first to Qth racks, wherein a first terminal, the Q×M unit cells mounted on the nth stages of the first to Qth racks and a second terminal are cascaded, each of Q, N, and M is an integer not smaller than 2, characterized in that each rack has N-1 wiring boards, or N-1 sets of wiring boards, arranged in respective N-1 gaps among the first to Nth stages; a wire, connecting a second terminal of an nth stage with a first terminal of an (n+1)th stage, is formed in each wiring board or in each set of wiring boards; and the QxNxM unit cells mounted on the first to Nth stages of the first to Qth racks and the wires of the wiring boards are helically cascaded to implement a reactor, or the QxNxM unit cells mounted on the first to Nth stages of the first to Qth racks and the wires of the sets of wiring boards are helically cascaded to implement a reactor. first and second DC connectors (3a, 3b) supplying and receiving DC power; an AC connector (3c-3e) supplying and receiving AC power; a first arm (A1-A3) connected between the first DC connector and the AC connector; and a second arm (A4-A6) connected between the AC connector and the second DC connector, each of the first and second arms including a plurality of cascaded unit cells (10, 10A, 10B), each unit cell having a switching element (S1, S2/S1-S4) and a capacitor, and at least some of the plurality of unit cells in at least one of the first and second arms being helically cascaded, wherein at least one of the first and second arms (A1-A3, A4-A6) includes first to Qth racks (LK1, LK2), each rack has first to Nth stages (ST1-ST5, ST11-ST15) sequentially disposed in a direction of height, M cascaded unit cells (10, 10A, 10B) are mounted on each stage, such that for each integer n not smaller than 1, and not larger than N, QxM unit cells are mounted on the nth stages of the first to Qth racks, wherein a first terminal, the Q×M unit cells mounted on the nth stages of the first to Qth racks and a second terminal are cascaded, each of Q, N, and M is an integer not smaller than 2, characterized in that each rack has N-1 wiring boards, or N-1 sets of wiring boards, arranged in respective N-1 gaps among the first to Nth stages; a wire, connecting a second terminal of an nth stage with a first terminal of an (n+1)th stage, is formed in each wiring board or in each set of wiring boards; and the QxNxM unit cells mounted on the first to Nth stages of the first to Qth racks and the wires of the wiring boards are helically cascaded to implement a reactor, or the QxNxM unit cells mounted on the first to Nth stages of the first to Qth racks and the wires of the sets of wiring boards are helically cascaded to implement a reactor.