A power grid control system using electric vehicles, comprising: charging/discharging spots (105, 107-109) which are connected to a power grid and which are adapted to charge storage batteries of electric vehicles connected thereto and discharge charged power of the storage batteries to the power grid; a data center which is adapted to collect and store current position information of electric vehicles located in a territory and charging state information of storage batteries of the electric vehicles; and a power aggregator which is adapted to rank degrees of necessity of charging/discharging the storage batteries of the respective electric vehicles based on the stored position information, the charging state information of the storage batteries and position information of the charging/discharging spots (105, 107-109) and create a ranking list indicating the ranks, wherein the power aggregator is adapted to distribute information for guiding each of the electric vehicles to a designated one of the charging/discharging spots (105, 107-109) for charging or discharging in accordance with the ranking list, characterized in that the power aggregator is adapted to calculate a charging/discharging power amount x from the storage battery of each electric vehicle so that the cost F(x) approximated by the quadratic function F(x) = a i x 2 + b i x + c i is optimized, wherein i is a number allocated to each electric vehicle, a i > 0 is set to correspond to a price suitability indicating coincidence of power purchase and sale prices between the power aggregator and each electric vehicle user, or a level of CO 2 emission, b i is set to correspond to the charging state of the storage battery mounted in each electric vehicle, and c i is set to correspond to a physical distance between the charging/discharging spot (105, 107-109) and the electric vehicle, or a temporal distance between the charging/discharging spot (105, 107-109) and the electric vehicle. charging/discharging spots (105, 107-109) which are connected to a power grid and which are adapted to charge storage batteries of electric vehicles connected thereto and discharge charged power of the storage batteries to the power grid; a data center which is adapted to collect and store current position information of electric vehicles located in a territory and charging state information of storage batteries of the electric vehicles; and a power aggregator which is adapted to rank degrees of necessity of charging/discharging the storage batteries of the respective electric vehicles based on the stored position information, the charging state information of the storage batteries and position information of the charging/discharging spots (105, 107-109) and create a ranking list indicating the ranks, wherein the power aggregator is adapted to distribute information for guiding each of the electric vehicles to a designated one of the charging/discharging spots (105, 107-109) for charging or discharging in accordance with the ranking list, characterized in that the power aggregator is adapted to calculate a charging/discharging power amount x from the storage battery of each electric vehicle so that the cost F(x) approximated by the quadratic function F(x) = a i x 2 + b i x + c i is optimized, wherein i is a number allocated to each electric vehicle, a i > 0 is set to correspond to a price suitability indicating coincidence of power purchase and sale prices between the power aggregator and each electric vehicle user, or a level of CO 2 emission, b i is set to correspond to the charging state of the storage battery mounted in each electric vehicle, and c i is set to correspond to a physical distance between the charging/discharging spot (105, 107-109) and the electric vehicle, or a temporal distance between the charging/discharging spot (105, 107-109) and the electric vehicle.