NOVELTY - The fuel cell system (1) has a controller (18) which is configured for controlling air supply amount by a first control amount based on detection concentration of an oxygen concentration sensor (21) in a decrease direction when the detection water level of a water level sensor (22) is lower than a predetermined water level. The fuel supply amount is controlled by a second control amount according to detection concentration of the oxygen concentration sensor in increase direction when the detected temperature of a temperature sensor (20) is lower than a predetermined temperature.    USE - Fuel cell system, e.g., water self-sustaining type fuel cell system, for generating electricity.    ADVANTAGE - Increases control of air supply amount and reduces air utilization factor small when the water level of water tank is higher than a set water level while improving stability of water self sustaining and reducing the maintenance cost sine the fuel supply amount is controlled by a second control amount according to the detection concentration of the Oxygen concentration sensor in increase direction when the detected temperature of the temperature sensor is lower than a predetermined temperature. Prevents generation of carbon monoxide and improves safety since the oxygen concentration of waste gas is monitored and the control amount is appropriately adjusted during increase and decrease control of air supply amount and fuel supply amount. Effectively shortens recovery time and reduces power consumption by increases control of air supply amount. Improves efficiency of water recovery from waste gas since the control amount of fuel supply amount are appropriately controlled according to the concentration of the reaction product of the fuel cell.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for a control method of the fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic block diagram of the fuel cell system. (Drawing includes non-English Language text).    Fuel cell system (1)    Controller (18)    Temperature sensor (20)    Oxygen concentration sensor (21)    Water level sensor (22)