There is provided a device including: an inverter 5 including switching elements 51 and 52 for outputting a U-phase voltage and switching elements 53 and 54 for outputting a W-phase voltage; midpoint capacitors 31 and 32; a voltage detector 4 configured to detect a condition of a midpoint potential in an O phase at a connecting point between the midpoint capacitor 31 and the midpoint capacitor 32; an AC voltage detector 8; a midpoint stabilizer 2 configured to regulate a power storage amount of each of the midpoint capacitor 31 and the midpoint capacitor 32; and a controller 7 configured to control the switching elements 51, 52, 53 and 54 so as to supply a desired U-phase voltage and a desired W-phase voltage based on voltages detected by the AC voltage detector 8, and control the midpoint stabilizer 2 so as to adjust the midpoint potential in the O phase based on the condition of the midpoint potential detected by the voltage detector 4.
