A controller installed at each consumer side comprises: a power generation amount acquisition unit acquiring the amount of electricity that has been generated in a solar cell; a power selling amount acquisition unit acquiring the amount of electricity for sale that has been allowed to flow into an electric power system by reverse power flow, among the electricity generated in the solar cell; and a power selling suppression control unit configured to set a threshold value for the amount of the electricity for sale, as proportional to the amount of the generated electricity acquired by the power generation amount acquisition unit, and configured to suppress the reverse power flow into the electric power system so that the amount of the electricity for sale is set to the threshold value or less.