When in the independent system: a natural energy generating apparatus, a storage battery system, a power generator, and a first load is connected to a power supply system; the natural energy generating apparatus supplies generated power to the power supply system; the storage battery system is a regulated power supply; and the power generator performs a constant power operation, information indicating states of the natural energy generating apparatus, the storage battery system, the power generator, the first load, and a second load is obtained. Operations of the natural energy generating apparatus and the power generator, connection of the second load to the power supply system, and paralleling off the second load from the power supply system are controlled based on the information so that a demand-supply balance of power in the independent system is maintained, with the first load maintained to be connected to the power supply system.