A first switching unit switches a first power grid from an isolated operation to a grid-connected operation. In switching the first power grid from the isolated operation to the grid-connected operation, the first switching unit determines a master and a slave among a plurality of power adjustment resources and allows the first power grid to be connected to the second power grid after master-slave control by means of the master and the slave such that electric power of the first power grid is in synchronization with electric power of the second power grid. The first switching unit preferentially selects as the master, a power adjustment resource close to a point of connection between the first power grid and the second power grid from among the plurality of power adjustment resources.