A contactor electrically connects and disconnects a fuel cell from a load. When a failure-detecting mode for detecting a closing failure of a contactor is initiated, an opening command is transmitted to the contactor, and a DC/DC converter connected to a motor changes the load voltage. Then, the load voltage and a fuel cell voltage are compared with each other. If the contactor is in a normal open state, as a result of the opening command, the fuel cell voltage is constant, whereas the load voltage of the DC/DC converter decreases, thereby producing a voltage difference. A closing failure of the contactor is determined when it is detected that the fuel cell voltage is substantially equal to the load voltage.