The invention judges a total power supply/demand condition of the electricity consumers 2 as a whole according to an individual power supply/demand condition of electricity consumers 2, judges a total usable capacity of all batteries 17 according to a usable capacity of the batteries 17 of electric vehicles 10 parked at electricity consumers 2, obtains required charge/discharge amounts of all the batteries 17 according to a result of comparison of the total power supply/demand condition of the electricity consumers 2 as a whole with the total usable capacity of all the batteries 17, subjects the batteries 17 to charge/discharge controls according to the required charge/discharge amounts, the power supply/demand conditions of the electricity consumers 2, and the usable capacities of the batteries.