When a request power for a fuel cell is smaller than a predetermined valu e, a fuel cell system stops supply of oxygen gas to a fuel cell and lowers t he output voltage of the fuel cell from a user upper limit voltage (V1) to a reduction voltage (V3) so as to activate a catalyst. When the output voltag e of the fuel cell is lowered to an air blow voltage (V4) because of insuffi cient oxygen gas, the fuel cell system supplies oxygen gas to recover the ou tput voltage of the fuel cell.
