A power conversion device in which an inverter for controlling a load is connected to an alternating current power system, and arranged to perform an electric power assist by connecting a direct-current power assist device having a chopper and a charge device to a direct-current circuit of the inverter. The device including a setting section to set charge and discharge target values in accordance with a sensed value of the direct-current voltage of the inverter; a charge control section to perform a charge control based on the charge target value; a discharge control section to perform a discharge control based on the discharge target value; and an instantaneous-low high-speed-compensation section to estimate an electric power corresponding to a direct-current sensed voltage of the inverter, and to output a value to the discharge control section which is obtained by dividing the estimated value by the direct-current sensed voltage.