NOVELTY - The fuel cell system comprises a fuel cell (6) that decomposes hydrocarbon-based fuel to produce an electrical energy, a fuel tank (1), from which the fuel is fed into the fuel cell, and a unit (3) for treatment of decomposition products into which decomposition products of fuel are directed. The fuel tank and treatment unit are separated from each other by a movable wall that is arranged to move, in order to even out a pressure difference and/or a volume difference, between the fuel tank and the treatment unit. The products are led to the treatment unit through a separation membrane (4).    USE - Used as a fuel cell system.    ADVANTAGE - The closed and gas-tight fuel cell system is inexpensive, exhibits improved current and power densities and less power losses and malfunctions, and does not require mechanical pumps or valves.    DETAILED DESCRIPTION - The fuel cell system comprises a fuel cell (6) that decomposes hydrocarbon-based fuel to produce an electrical energy, a fuel tank (1), from which the fuel is fed into the fuel cell, and a unit (3) for treatment of decomposition products into which decomposition products of fuel are directed. The fuel tank and treatment unit are separated from each other by a movable wall that is arranged to move, in order to even out a pressure difference and/or a volume difference, between the fuel tank and the treatment unit. The products are led to the treatment unit through a separation membrane (4), which is selective with regard to the decomposition products to be treated. The fuel cell comprises fuel input nozzles connected to the fuel tank, and outlet nozzles connected to the treatment unit. The separation membrane is arranged between the outlet nozzles and the treatment unit and arranged to remove the decomposition products that are to be left untreated in the treatment unit from a flow of the decomposition products directed into the treatment unit. The fuel tank, the treatment unit and an anode space of the fuel cell together form a closed unit, which is gas-and liquid-tight. A structure of the fuel tank decomposes the hydrocarbon-based fuel by using direct oxidation in the anode space of the fuel cell. The decomposition products of the fuel are comprised of carbon dioxide and water. The treatment unit comprises: an agent reacting with the decomposition products that are generated in the fuel cell such that liquid or solid compounds are generated from the decomposition products; an agent that reacts with the carbon dioxide and transforms the carbon dioxide into the liquid or solid compound; and a tank made of polymer material. A tank wall of treatment opposite to the fuel tank is made of a flexible material. The fuel cell, the fuel tank and the treatment unit are connected in series such that the fuel cell is placed in middle in a flow direction of the fuel and the decomposition products. The tank of the treatment unit moves as a function of the pressure difference and the volume difference between the liquid and/or gas space between the treatment unit and the fuel tank. The fuel tank and the treatment unit have a shared wall, which is made of a flexible material. An unobstructed access from the fuel tank to the fuel cell is present such that a quantity of feed of the fuel varies according to the pressure difference or the volume difference or the pressure difference and the volume difference between the fuel tank and the treatment unit. The treatment unit comprises an alkaline agent such as a solid material and a solution absorbed into a carrier such as porous paper or cardboard.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic cross-sectional view of a fuel cell system.    Fuel tank (1)    Movable wall (2)    Decomposition treatment unit (3)    Separation membrane (4)    Fuel cell. (6)