An energy supply system for supplying, in system interconnection, power received at a power receiving equipment (8) from a power plant and power generated by a fuel cell to a power consuming installation (3), and supplying heat generated by the fuel cell to a heat consuming installation (6). This system includes an operation amount computing device for computing an amount of operation of the fuel cell to minimize an equation y = a . L + b . M + c . N, in response to an energy demand of the power consuming installation (3) and heat consuming installation (6). A control device controls the fuel cell to satisfy the amount of operation computed. The system supplies energy in optimal conditions with respect to the cost borne by an energy consumer, consumption of primary energy, and release of environmental pollutants.; Energy is effectively used from the standpoint of the energy consumer and a national point of view.