Disclosed is a fuel cell system which includes a fuel cell having an anode and a cathode and for producing electricity through reaction between the fuel gas and the oxidant gas, a fuel gas supply path through which the fuel gas passes, an oxidant gas supply path through which the oxidant gas passes, a voltage sensing device for detecting voltage produced by the fuel cell during power generation, and a decision device for determining whether or not cross leakage occurs between the anode and the cathode of the fuel cell, based on voltage information sent from the voltage sensing device. Moreover, the decision device determines that the cross leakage occurs between the anode and the cathode, if the voltage produced by the fuel cell is less than a predetermined value during the power generation.