To provide a fuel cell system capable of reducing fuel loss caused by crossover and realizing high energy density.

The fuel cell system is equipped with a fuel cell 1, a dilution fuel container 2 filled with methanol aqueous solution, a fuel supply passage 3 for supplying the methanol aqueous solution to the fuel cell 1, a circulation passage 4b for circulating the methanol aqueous solution not consumed in the fuel cell 1 to the dilution fuel container 2, a replenishing fuel container 5 for storing replenishing methanol, a fuel replenishing passage 6 for supplying methanol to the fuel supply passage 3, a valve 7 installed in the fuel replenishing passage 6, a concentration sensor 9 for detecting the concentration of the methanol aqueous solution, a fluid pump 10 installed in the fuel supply passage 3, a concentration control part 13a for controlling the concentration of the methanol aqueous solution in a prescribed value by controlling opening and closing of the valve 7 based on the output of the concentration sensor 9, and a control circuit 13 having a flow rate control part 13b for supplying the methanol aqueous solution having constant flow rate to the fuel cell 1 by controlling the fluid pump 10.

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