A power converter has a series direct connection mode of keeping on/off of a plurality of switching elements to maintain the state where first and second DC power supplies different in amount of voltage change with respect to input/output of the same amount of electric power are connected in series with an electric power line connected to a load, and a voltage controlling mode of controlling an output voltage on the electric power line to be a voltage command value by controlling on/off of the plurality of switching elements. In the voltage controlling mode, between time tx and ta, the sum of voltages of the first and second DC power supplies is matched with the voltage command value by controlling the output voltage by means of charging/discharging between the first and second DC power supplies. After time ta, the series direct connection mode is applied.