A DC/DC conversion device comprising: two DC bus-bars (8A, 8B) electrically connected to DC power-supply terminals (A, B) configured to receive an input DC voltage; a first and a second capacitor (10A, 10B) connected in series between the DC bus-bars (8A, 8B), wherein an electrical connection point between the capacitors (10A, 10B) is led out as an intermediate potential terminal to a power-conversion circuit unit; the power conversion circuit unit comprising three-level power conversion circuits (12a-12f) configured as a plurality of phases, each of the three-level power conversion circuits (12a-12f) being connected in parallel to the capacitors (10A, 10B) that are connected in series and being configured for converting the input DC voltage to three levels of potential, namely a high potential, the intermediate potential, and a zero potential, wherein the three-level power conversion circuits (12a-12f) are connected on their output side to a load in parallel; voltage detectors (21, 22) that are configured to detect at least two of the input DC voltage, a first voltage (EfcH) of the first capacitor, and a second voltage (EfcL) of the second capacitor; and a voltage control unit that is configured to control an output voltage of the three-level power conversion circuits (12a-12f) on the basis of a voltage command value, wherein at least one of the phases is configured to operate as an imbalance-reduction phase that executes imbalance-reduction control such that one of the first and second voltages has a value that is half of the input DC voltage, the three-level power conversion circuits (12a-12f) except for the imbalance-reduction phase are configured to operate as power conversion phases that perform DC-to-DC power conversion, and the voltage control unit includes an imbalance-reduction control unit that is configured to control an output voltage of the imbalance-reduction phase by using two of the voltages that are detected by the voltage detectors (21, 22). two DC bus-bars (8A, 8B) electrically connected to DC power-supply terminals (A, B) configured to receive an input DC voltage; a first and a second capacitor (10A, 10B) connected in series between the DC bus-bars (8A, 8B), wherein an electrical connection point between the capacitors (10A, 10B) is led out as an intermediate potential terminal to a power-conversion circuit unit; the power conversion circuit unit comprising three-level power conversion circuits (12a-12f) configured as a plurality of phases, each of the three-level power conversion circuits (12a-12f) being connected in parallel to the capacitors (10A, 10B) that are connected in series and being configured for converting the input DC voltage to three levels of potential, namely a high potential, the intermediate potential, and a zero potential, wherein the three-level power conversion circuits (12a-12f) are connected on their output side to a load in parallel; voltage detectors (21, 22) that are configured to detect at least two of the input DC voltage, a first voltage (EfcH) of the first capacitor, and a second voltage (EfcL) of the second capacitor; and a voltage control unit that is configured to control an output voltage of the three-level power conversion circuits (12a-12f) on the basis of a voltage command value, wherein at least one of the phases is configured to operate as an imbalance-reduction phase that executes imbalance-reduction control such that one of the first and second voltages has a value that is half of the input DC voltage, the three-level power conversion circuits (12a-12f) except for the imbalance-reduction phase are configured to operate as power conversion phases that perform DC-to-DC power conversion, and the voltage control unit includes an imbalance-reduction control unit that is configured to control an output voltage of the imbalance-reduction phase by using two of the voltages that are detected by the voltage detectors (21, 22).