A fuel cell system has a plurality of fuel cells stacked in one or more groups of fuel cells. Each fuel cell includes a fuel electrode supplied with fuel gas at a fuel gas supply pressure, an oxidizing electrode supplied with oxidizing gas at an oxidizing gas supply pressure, and an electrolyte membrane disposed between the fuel electrode and the oxidizing electrode. A pressure-difference control unit generates a pressure difference across the membrane such that the fuel gas supply pressure is greater than the oxidizing gas supply pressure in each fuel cell, a cell-voltage measuring device measures a cell voltage for each fuel cell or each group of fuel cells in the fuel cell stack, and a leakage determination unit determines the presence or absence of a leaking cell based on the behavior of the cell voltage of each fuel cell while the pressure difference is increased with time.