NOVELTY - A fuel cell system comprises fuel cell stack (110); fuel concentration sensor (143) installed in a fuel mixing tank (130), to measure concentration of aqueous fuel supplied to the stack; a load amount measuring unit to measure load amount on the stack; a temperature estimating unit (152) to estimate temperature at the sensor from the measured load amount; a concentration correcting unit (154) to produce corrected concentration value by applying the estimated temperature to sensing values of sensor; and drive controller (150) to control drive of system according to corrected concentration.    USE - As a fuel cell system e.g. direct methanol fuel cell system (claimed), and systems including phosphoric acid fuel cell, molten carbonate fuel cell, solid oxide fuel cell, polymer electrolyte membrane fuel cell or alkaline fuel cell; useful for producing electricity/power.    ADVANTAGE - The fuel cell system does not directly control the fuel concentration in the mixing tank, from the concentration sensing values and the load amount on the stack, but further estimates the temperature of the mixing tank to correct the concentration sensing values. Therefore, the fuel cell system corrects concentration sensing values without using a separate temperature sensor, thus ensuring an accurate operation of the fuel cell system at low cost and small volume. The fuel cell system improves the accuracy of the concentrations measured by the fuel concentration sensor, without using additional components. The concentrations of the mixed fuel in the mixing tank is constantly kept within a range of concentrations, enabling improved operation of the fuel cell system. The fuel cell system is controlled in a convenient manner.    DETAILED DESCRIPTION - A fuel cell system comprises a fuel cell stack (110); a fuel tank (190) storing a high-concentration fuel; a fuel mixing tank (130) for mixing high-concentration fuel from fuel tank with condensed water, to generate an aqueous fuel solution; a fuel concentration sensor (143) installed in the fuel mixing tank, to measure concentration of the aqueous fuel solution supplied to the fuel cell stack; a load amount measuring unit to measure electrical load amount on the fuel cell stack; a temperature estimating unit (152) to estimate a temperature at the concentration sensor from the measured load amount value; a concentration correcting unit (154) to produce a corrected concentration value by applying the estimated temperature to the sensing values of the concentration sensor; and a drive controller (150) for controlling a drive of the fuel cell system according to the corrected concentration, to control concentration of the fuel. The fuel cell system further comprises a power converter (180); and a heat exchanger (160). The drive controller controls fuel supply from the fuel tank to the fuel mixing tank and/or controls the degree of condensation of the heat exchanger. The fuel cell stack generates power/electricity by electrochemically reacting a fuel with an oxidant. The load amount measuring unit preferably measures voltage and/or current at the installed point. The temperature estimating unit preferably includes a table containing load amount values and corresponding estimated temperatures. The concentration correcting unit preferably includes a table containing temperatures, and corresponding corrected concentration values. An INDEPENDENT CLAIM is included for controlling the fuel cell system, involving measuring a load amount value on the stack of fuel system; estimating a temperature at a concentration sensor from the load amount value; producing a corrected concentration value by correcting a concentration sensing value from the concentration sensor according to the estimated temperature; and controlling a drive of the fuel cell system according to the corrected concentration value.    DESCRIPTION OF DRAWING(S) - The figure shows a schematic view of a fuel cell system.    Fuel cell stack (110)    Conduits (122, 124, 126, 128)    Fuel mixing tank (130)    Ammeter (142)    Fuel concentration sensor (143)    Pump (148)    Drive controller (150)    Temperature estimating unit (152)    Concentration correcting unit (154)    Heat exchanger (160)    Power converter (180)    Fuel tank (190)    External load. (200)