NOVELTY - A fuel cell system comprises a pressure control range calculating unit that calculates a controllable range of the pressure control unit on the basis of the detected flow rate and/or the detected pressure, where the flow rate control unit controls the flow rate of reaction gas on the basis of the calculated pressure controllable range.    USE - Used in vehicle, ships, aircraft, and fixed type apparatuses for home use and industrial use.    ADVANTAGE - Continuously controlling the pressure toward the target pressure can prevent an excess pressure in the fuel cell, so that, disadvantages such as accelerated deterioration of the membrane in the fuel cell can be prevented. The fuel cell system achieves accurate control of the air pressure toward the target pressure without decrease in the durability of the fuel cell.    DETAILED DESCRIPTION - A fuel cell system comprises:    (1) a fuel cell that generates an electric power by a reaction of a reaction gas;    (2) a flow rate detector that detects a flow rate of the reaction gas supplied to the fuel cell;    (3) a pressure detector that detects a pressure of the reaction gas supplied to the fuel cell;    (4) a flow rate control unit that controls the flow rate of the reaction gas;    (5) a pressure control unit that controls the pressure of the reaction gas; and    (6) a pressure control range calculating unit that calculates a controllable range of the pressure control unit on the basis of the detected flow rate and/or the detected pressure, where flow rate control unit controls the flow rate on the basis of the calculated pressure controllable range.    An INDEPENDENT CLAIM is also included for controlling a fuel cell system, comprising:    (A) generating an electric power by a reaction of a reaction gas;    (B) detecting a flow rate of the reaction gas supplied to the fuel cell;    (C) detecting a pressure of the reaction gas supplied to the fuel cell;    (D) controlling the flow rate of the reaction gas;    (E) controlling the pressure of the reaction gas; and    (F) calculating a controllable range of the pressure on the basis of the detected flow rate and/or pressure, where flow rate is controlled on the basis of the calculated pressure controllable range.    DESCRIPTION OF DRAWING(S) - The figure is a block diagram of a fuel cell system.