Output voltage of a fuel cell 2 is decreased by a converter 51 to conduct an activation treatment to catalyst of the fuel cell 2, while measuring reduction current by a current sensor 2a while scanning output voltage of the fuel cell 2 over a certain range by the converter 51 as measurement of cyclic voltammetry under the condition that supply of oxidation gas to the fuel cell 2 is stopped from a compressor 31, and this measurement value is integrated by a control device 6. The control device 6 finds a charge amount of electrode catalyst of the fuel cell 2 based on this integration value, decides whether this charge amount is smaller or not than a degradation decision value, and displays this decision result on a display 55. A decision can be made precisely as to whether the electrode catalyst of the fuel cell is degraded or not.