NOVELTY - The system (2) has an air humidifier (46) that moistens a cathode electrode (18) at the time of operation stop of a fuel cell (3). An electronic control unit (ECU) (51) judges whether the electric power generation in the fuel cell is stopped or not so as to control the operation of moisture unit, an air supply valve (44), and an air exhaust valve (45) so that the cathode electrode is moistened by the air humidifier and the air supply and exhaust valves are displaced to a closed state when the electric power generation in fuel cell is judged to be stopped.    USE - System of fuel cell such as solid polymer form fuel cell mounted in vehicle such as electric vehicle using liquid fuel such as methanol, dimethyl ether, and hydrazine.    ADVANTAGE - The ECU judges whether the electric power generation in the fuel cell is stopped or not so as to control the operation of moisture unit, air supply valve, and air exhaust valve so that the cathode electrode is moistened by the air humidifier and the air supply and exhaust valves are displaced to a closed state when the electric power generation in fuel cell is judged to be stopped. Thus, the cathode electrode is maintained effectively in moist state at the time of stopping of fuel cell. Hence, the precipitation of salt in the fuel cell system is suppressed by suppressing drying of cathode electrode while suppressing the deterioration of cathode electrode and damage of fuel cell system. The air passing through the air supply line is humidified when the air humidifier operates to the electric power generation time of fuel cell so that the humidified air is supplied to the cathode electrode so as to make the cathode electrode is made into the moist state, and the electrolytic membrane is made into the moist state through the cathode electrode. Thus, the electroconductive of the electrolytic membrane is improved while improving the generating efficiency of fuel cell.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of the electric vehicle with the mounted fuel cell. (Drawing includes non-English language text)    Fuel cell system (2)    Fuel cell (3)    Cathode electrode (18)    Air supply valve (44)    Air exhaust valve (45)    Air humidifier (46)    ECU (51)