An electric power system in which a plurality of electric power suppliers and demanders are mutually connected via control devices is self-sustainable but also capable of coexisting with the known electric power system. Suppliers/demanders 11 to 15 are mutually connected via an electric power supply and demand line W and have power generation devices 101 and 151, electrical storage devices 102 and 152, a plurality of loads 103, and electric power supply and demand control devices 104 and 153 that detect an electricity shortage or surplus, receives or delivers electric power from or to other suppliers/demanders 12 to 15 accordingly, and automatically or manually controls the power generation devices including a co-gene in respective suppliers/demanders on the basis of a weather forecast, demand prediction, and the like.