NOVELTY - A fuel cell system has a fuel cell, a fuel-gas supply unit (21), an oxidizing-agent gas supply unit (20) and a gas supply control unit (401). The fuel cell has fuel electrode and oxidizing agent electrode, arranged on both sides of electrolyte film. The gas supply control unit controls the supply units, such that relationship between hydrogen amount and oxygen amount in boundary where electrolyte film touches the oxidizing-agent gas electrode at the time of idle running of fuel cell system or open-end voltage maintenance, is specified.    USE - As drive motive power source for motor vehicles.    ADVANTAGE - The fuel cell system suppresses hydrogen peroxide production by oxygen amount reduction by fuel electrode catalyst, and suppresses attack of electrolyte film by hydrogen peroxide.    DETAILED DESCRIPTION - A fuel cell system has a fuel cell, a fuel-gas supply unit (21), an oxidizing-agent gas supply unit (20) and a gas supply control unit (401). The fuel cell generates electric power according to electrochemical reaction of fuel gas and oxidizing agent gas. The fuel cell is equipped with a fuel electrode and an oxidizing agent electrode, arranged on both sides of an electrolyte film. The fuel-gas supply unit supplies fuel gas containing hydrogen to fuel electrode, and the oxidizing-agent gas supply unit supplies oxidizing-agent gas containing oxygen to the oxidizing-agent gas electrode. The gas supply control unit controls the supply amount of fuel gas and oxidizing agent gas by their respective supply units, such that relationship between hydrogen amount and oxygen amount in the boundary where electrolyte film touches the oxidizing-agent gas electrode at the time of idle running of fuel cell system or open-end voltage maintenance is shown by a formula: (hydrogen amount-2xoxygen amount)greater than 0.    DESCRIPTION OF DRAWING(S) - The figure shows the block diagram of fuel cell system. (Drawing includes non-English language text)    Oxidizing-agent gas supply unit (20)    Fuel gas supply unit (21)    Filter (201)    Circulation compressor (216)    Control unit (401)