An electrical leakage determining circuit determines the occurrence of electrical leakage at a negative electrode of a fuel cell when a potential difference V between a positive electrode of the fuel cell and ground, which is detected by a potential difference detecting circuit, exceeds a preset potential difference threshold Vth, and determines the occurrence of electrical leakage at the positive electrode of the fuel cell when an insulation resistance R between the positive electrode of the fuel cell and ground, which is detected by a resistance detection circuit, falls below a resistance threshold Rth preset to be equal to or smaller than a resistance value Rw of a coolant. Upon determination of the occurrence of electrical leakage, the electrical leakage determining circuit opens a switching contact provided on a power supply line to interrupt power supply from the fuel cell to various power consumption components.