NOVELTY - A fuel cell system comprises a fuel cell stack including a hydrogen electrode and an oxygen electrode, generation control portion that causes the fuel cell stack to generate an electric power corresponding to requested electric power, and non-generation-time control portion that operates the oxygen supplier portion and/or hydrogen supplier portion based on predetermined condition regardless of requested    USE - The system is used for generating electric power through electrochemical reactions between hydrogen and oxygen. It is used as energy source of a mobile unit (all claimed) such as electric vehicle.    ADVANTAGE - The system can output a requested electric power at good timing even when the operation mode is switched from the intermittent operation mode to the normal operation mode. Thus, drivability can be improved.    DETAILED DESCRIPTION - A fuel cell system that generates electric power when supplied with hydrogen and oxygen, comprises:    (a) a fuel cell stack including a hydrogen electrode and an oxygen electrode that are disposed at opposite sides of an electrolyte;    (b) hydrogen supplier portion that supplies hydrogen to the hydrogen electrode;    (c) oxygen supplier portion that supplies oxygen to the oxygen electrode;    (d) input portion that inputs a requested electric power;    (e) generation control portion that causes the fuel cell stack to generate an electric power corresponding to the requested electric power by controlling the oxygen supplier portion and the hydrogen supplier portion; and    (f) non-generation-time control portion that stops a generation control performed by the generation control portion if the requested electric power is at most a predetermined value, and that operates at least one of the oxygen supplier portion and the hydrogen supplier portion based on a predetermined condition regardless of the requested electric power.    INDEPENDENT CLAIMS are also included for:    (a) a mobile unit capable of moving by an electric motor that is driven by a fuel cell system as energy source; and    (b) a control method for a fuel cell system that generates electric power when supplied with hydrogen and oxygen, comprising inputting a requested electric power; causing the fuel cell stack to generate an electric power corresponding to the requested electric power by controlling the oxygen supplier portion and the hydrogen supplier portion; stopping electric power generation by the fuel cell stack if the requested electric power is at most a predetermined value; and operating at least one of the oxygen supplier portion and the hydrogen supplier portion based on a predetermined condition regardless of the requested electric power after the electric power generation is stopped.    DESCRIPTION OF DRAWING(S) - The figure is a diagram showing an overall construction of fuel cell system.    Filter (40)    Compressor (41)    Humidifier (42)    Diluter (44)    Pressure sensor (51)