NOVELTY - An electric current extraction unit (30) controls the voltage value when extracting electric current from a fuel cell stack (1). A main control unit (31) controls the opening and closing of an inlet valve (23) and an outlet valve (24) and the electric current extraction by the electric current extraction unit. The main control unit operates the electric current extraction unit during a system stop to extract electric current. The inlet and outlet valves are closed after the start of electric current extraction.    USE - Fuel cell system for use in vehicle as power supply of electric motor that drives vehicle.    ADVANTAGE - Operates electric current extraction unit during system stop to extract electric current, thus oxidizing agent gas at side of oxidizing agent electrode of fuel cell stack may be consumed. Controls inflow of external air during system stop period with inlet and outlet valves, thus suppressing increase in oxidizing agent gas concentration at side of oxidizing agent electrode at system start-up and preventing deterioration reaction of oxidizing agent gas at oxidizing agent electrode side. Eliminates need to extract electric current during starting since increase in oxidizing agent gas during system start-up is suppressed, thus preventing reduction in deterioration inhibitory effect during fuel cell system start-up and preventing starting time from taking too long.    DETAILED DESCRIPTION - The fuel cell stack has an oxidizing agent electrode and a fuel electrode that receive a supply of oxidizing agent gas and fuel gas, and generates electric power by making fuel gas and oxidizing agent gas react. The inlet valve and outlet valve regulate inflow of external air at the side of the oxidizing agent electrode through valve closure.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram of the fuel cell system. (Drawing includes non-English language text).    Fuel cell stack (1)    Inlet valve (23)    Outlet valve (24)    Electric current extraction unit (30)    Main control unit (31)