In a variable speed generator/motor device including a variable frequency power converter, a direct current voltage device including a voltage type self-excited converter, an automatic voltage adjuster, and a converter current adjuster that controls unit converters, a first three-phase branch circuit is provided between the direct current voltage device and an alternating current system; a second three-phase branch circuit is provided between the variable frequency power converter and a three-phase alternating current synchronous machine; a first load switch is provided between the first three-phase branch circuit and the second three-phase branch circuit; a measurement current transformer is provided between the three-phase alternating current synchronous machine and the second three-phase branch circuit; when switching from a converter mode in which the variable frequency power converter drives the three-phase alternating current synchronous machine to generate power to a bypass mode, the first load switch is closed to stop a gate command to the unit converters; and when switching from the bypass mode to the converter mode, a current command value of the converter current adjuster is calculated from a current value of the measurement current transformer, the gate command to the unit converters is started, and the first load switch is opened.