NOVELTY - A fuel cell system comprises a fuel cell stack (210) to generate electric power through an electrochemical reaction of hydrogen and oxygen; a mixing tank (230) to generate a diluted fuel by mixing a raw fuel with water discharged from fuel cell stack; a reference concentration tank (290) to store a predetermined optimum concentration of a reference solution for fuel cell stack; and a concentration sensing module (220) to measure a concentration of the reference solution or diluted fuel supplied from mixing tank to the fuel cell stack.    USE - As fuel cell system (claimed), e.g. direct methanol fuel cell system used in static power plants of housing and public buildings, transportable power sources, such as portable electronic equipment, and movable power sources such as automobile power sources.    ADVANTAGE - The fuel cell system is capable of exactly controlling a concentration of a fuel supplied for generation of electricity regardless of deterioration of a concentration sensor with time. It is possible to secure a stable operation of the fuel cell system for an extended time by making up for the changes in the sensing characteristic of the concentration sensor by employing a separate concentration reference solution in the fuel cell system. The concentration comparator/controller does not use the measured concentration value used as a standard of judgment of the feedback control as is, and therefore it is possible to solve the problem regarding the deteriorated sensing characteristic of the concentration sensor using the compensation value calculated in the concentration correction mode. The fuel cell system can compensate for the deviation of the concentration sensor that is caused with the passage of time.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel cell stack (210) to generate electric power through an electrochemical reaction of hydrogen and oxygen; a mixing tank (230) to generate a diluted fuel by mixing a raw fuel with water discharged from fuel cell stack; a reference concentration tank (290) to store a predetermined optimum concentration of a reference solution for fuel cell stack; and a concentration sensing module (220) to measure a concentration of either the reference solution or diluted fuel supplied from mixing tank to the fuel cell stack. The fuel cell system further comprises a concentration comparator/controller (250) to compare the results obtained by measuring concentrations of the diluted fuel and the reference solution, and to control influx amounts of fluids flowing into the mixing tank; a condenser (270) to condense a cathode effluent of the fuel cell stack; fuel tank (295) to store the raw fuel; and fuel pump to transfer the raw fuel to the mixing tank, the concentration comparator/controller controlling pumping of the fuel pump. An INDEPENDENT CLAIM is included for driving a fuel cell system involving using a concentration sensor to maintain a constant concentration of a diluted fuel supplied to a fuel cell stack: operating the fuel cell system in a normal mode while counting accumulated use time; operating the fuel cell system in a concentration sensing mode to calculate a compensation value of a concentration sensor in response to the counted accumulated use time is equal to reference time; and applying the compensation value to a concentration sensing value of the concentration sensor.    DESCRIPTION OF DRAWING(S) - The figure shows a block diagram of direct methanol fuel cell system.    Fuel cell stack (210)    Sensing module (220)    Mixing tank (230)    Concentration comparator/controller (250)    Condenser (270)    Reference concentration tank (290)    Fuel tank (295)