NOVELTY - The fuel supply unit (10) comprises a fuel storage chamber (12) to house a stored fuel having a first concentration of fuel, a fuel recovery chamber (16) to allow passage of a recovered fuel having a second concentration of fuel, and a fuel permeable membrane (15) between the fuel storage chamber and the fuel recovery chamber. The second concentration is lower than the first concentration. The fuel permeable membrane is configured to pass the stored fuel to the fuel recovery chamber. The fuel storage chamber has a pouch or bellows shape.    USE - The unit is useful for supplying fuel to a stack of a fuel cell system such as a direct methanol fuel cell, where the fuel is methanol, ethanol, liquefied petroleum gas, liquefied natural gas, gasoline, and butane gas.    ADVANTAGE - The fuel supply unit is capable of economically and stably supplying the fuel to the stack of the fuel cell system less power consumption and easily controlling the concentration of fuel in the system.    DETAILED DESCRIPTION - The fuel supply unit (10) comprises a fuel storage chamber (12) to house a stored fuel having a first concentration of fuel, a fuel recovery chamber (16) to allow passage of a recovered fuel having a second concentration of fuel, and a fuel permeable membrane (15) between the fuel storage chamber and the fuel recovery chamber. The second concentration is lower than the first concentration. The fuel permeable membrane is configured to pass the stored fuel to the fuel recovery chamber. The fuel storage chamber has a pouch or bellows shape. The fuel permeable membrane comprises a perfluorosulfonic acid membrane. The fuel recovery chamber comprises grooves through which the recovered fuel flows. The grooves comprise a serpentine groove shape. The fuel storage chamber comprises a first fuel storage chamber. The fuel supply unit further comprises a concentration controller comprising a second fuel storage chamber. The second fuel storage chamber is configured to receive the stored fuel from the first fuel storage chamber. The fuel permeable membrane is between the second fuel storage chamber and the fuel recovery chamber. An INDEPENDENT CLAIM is included for a fuel cell system.    DESCRIPTION OF DRAWING(S) - The diagram shows a schematic view of a fuel cell system.    Fuel supply unit (10)    Fuel storage chamber (12)    Fuel permeable membrane (15)    Fuel recovery chamber (16)    Oxidant supply unit (20)    Fuel cell stack. (30)