A fuel cell generates electric power by circulating diluted fuel in a fuel cell. In a circulation dilution fuel cell system, a path is provided for leading vapor generated at an air pole of a fuel cell to a dilution fuel tank which circulates the diluted fuel of a fuel cell. Or, a fuel supply tank is provided in the upper position of the dilution fuel tank, so that the liquid fuel can be supplied to the dilution fuel tank by means of gravity. Thus, a pump for supplying the liquid fuel can be eliminated, and power consumption for controlling the fuel cell can be reduced.