NOVELTY - A contactor failure detecting apparatus comprises a contactor that is opened and closed to electrically connect and disconnect the fuel cell (14) with respect to the load, and a control device that transmits an opening command to a contactor while the contactor is in closed state, and changes the voltage or current on the load side of a direct current converter, while monitoring a change in at least one of a voltage, a current, and an electrical power on at least one of the fuel cell side and the load side of the contactor, for detecting a closing failure of the contactor.    USE - For a fuel cell system for a fuel cell vehicle.    ADVANTAGE - The apparatus is capable of detecting failure of a contactor disposed between the fuel cell and a load supplied with electrical energy from the fuel cell. The contactor acts to both connect and disconnect the load from the fuel cell. A closing failure of the contactor can be detected quickly. A long down time of the fuel cell system, as a result of detecting failure of the contactor, can be prevented. Overloading of the fuel cell can be prevented, thus avoiding degradation in performance of the fuel cell.    DETAILED DESCRIPTION - A contactor failure detecting apparatus comprises a fuel cell to generate power by reaction of a fuel gas and an oxygen-containing gas. A load is driven by an output from fuel cell. A contactor is opened and closed to electrically connect and disconnect the fuel cell with respect to the load. It has one end connected to an output side of the fuel cell and another end connected to the load. A direct current (DC/DC) converter capable of voltage variable control or current variable control, has one end connected to a load side of the contactor and another end connected to an energy storage device. A control device transmits an opening command to the contactor while the contactor is in closed state, and changes the voltage or current on the load side of the converter, while monitoring a change in at least one of a voltage, a current, and an electrical power on at least one of the fuel cell side and the load side of the contactor, for detecting a closing failure of the contactor.    DESCRIPTION OF DRAWING(S) - The figure is a circuit diagram of a fuel cell vehicle.    Fuel cell (14)    Air discharge port (26)    Condenser (52)    Voltmeter (102)    Diode (108)