The present invention detects a failure in an FC converter. A target voltage determination section determines an output target voltage for a fuel cell. A superimposition signal generation section generates a predetermined reference signal to be superimposed onto the output target voltage. A voltage command signal generation section generates a voltage command signal by superimposing the reference signal onto the output target voltage. A frequency characteristics calculation section calculates the frequency characteristics of the reference signal component superimposed on the output voltage of the fuel cell. A failure judgment section judges that a failure occurs in the FC converter if a value of the calculated frequency characteristics is less than the lower limit threshold value of an allowable range established based on reference characteristics. A gain adjustment section adjusts the gains in the FC converter so that the frequency characteristics calculated by the frequency characteristics calculation section correspond to the reference characteristics.