There is provided a V2G system including a parent aggregator which manages buying and selling of power from a plurality of power supply sources to a power grid, and a child aggregator which manages a plurality of transport machines and guarantees the parent aggregator charging and discharging between at least one of batteries mounted on the transport machines and the power grid. The child aggregator acquires first information regarding the charging and discharging and second information regarding the transport machines from each of the transport machines, aggregates the first information and the second information, and transmits at least the first information to the parent aggregator.