NOVELTY - Fuel cell system comprises fuel cell having controller provided with power production effective area estimating unit for estimating areas of electrodes available for producing power as power production effective area; current density calculating unit for calculating current density of the power production effective area; gas supply amount determining unit for determining amount of reactive gas to be supplied to the fuel cell based on the calculated current density; and gas supply controlling unit for controlling the supply unit to supply the amount of reactive gas.    USE - Used as fuel cell system.    ADVANTAGE - Fuel cell system is capable of limiting an extent of deterioration of the fuel cell even if part of a membrane component of the fuel cell is unavailable to produce power. The supply of reactive gases only to areas of electrodes available for power production in cryogenic environment prevents materials contained in catalyst from being consumed, which allows for power to be produced in a subfreezing environment while the fuel cell is protected.    DETAILED DESCRIPTION - The fuel cell system comprises fuel cell having anode and cathode sandwiching a membrane, and produces electric power from reaction of reactive gases via the membrane when the reactive gases are supplied to anode and cathode; supply unit for supplying the reactive gases to the fuel cell; and controller for controlling the supply unit. The controller comprises power production effective area estimating unit for estimating areas of the electrodes available for producing power as a power production effective area; current density calculating unit for calculating current density of the power production effective area based on a total power required by the fuel cell; gas supply amount determining unit for determining an amount of reactive gas to be supplied to the fuel cell based on the calculated current density; and gas supply controlling unit for controlling the supply unit to supply the amount of reactive gas. An INDEPENDENT CLAIM is included for a control method for a fuel cell comprising estimating areas of electrodes available for producing power as a power production effective area; calculating current density of the calculated power production effective area based on a total power required by the fuel cell; determining an amount of reactive gas to be supplied to the fuel cell based on the calculated current density; and supplying the determined amount of reactive gas to the fuel cell.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic diagram of the fuel cell system.