An AC impedance meter measures an AC impedance of fuel cells against at least a specific frequency. A sensor group detects at least one operating condition of the fuel cells. A detection unit compares a degradation reference value corresponding to the detected operating condition with a resistance value led from the measured AC impedance and detects performance degradation in the fuel cells based on a result of the comparison. This arrangement enables easy detection of performance degradation in the fuel cells, irrespective of a variation in operating condition of the fuel cells.