NOVELTY - The fuel cell system has a compressor (200) that is configured to compress air supplied to a fuel cell stack (100). A valve (600) is installed in an air supply pipe (300) connected from compressor to the stack. A bypass pipe (700) branches off at valve, and is connected to an exhaust pipe (320) of the stack. A controller (800) increases a pressure of the air supplied from compressor. The portion of the air is exhausted through bypass pipe by operating valve in response to determining to elevate the pressure of air supplied from compressor to pressure over a point on a surge curve.    USE - Polymer electrolyte membrane fuel cell system used for motor vehicle. Uses include but are not limited to passenger automobile such as sports utility vehicle, bus, truck, various commercial vehicles, watercraft such as boat and ship, aircraft, and gasoline-powered and electric-powered hybrid vehicle, electric vehicle, plug-in hybrid electric vehicle, hydrogen-powered vehicles and other alternative fuel vehicle.    ADVANTAGE - The durability and performance of the fuel cell can be improved by maintaining the condition in which the optimum water content in the electrode-membrane is provided. The performance of fuel cell can be maintained effectively by adjusting the humidity of air supplied to the fuel cell, even at high temperature operation condition for operating the fuel cell system. The performance of the fuel cell can be maintained even at a severe operation condition such as high temperature, high speed, and gradeability condition. The pressure and flow rate of air supplied to the fuel cell stack can be adjusted and surging and choking conditions of the compressor can be avoided, such that the more effective and rapid control of the humidity of air supplied to the fuel cell can be provided. Since the minimum operation curve can be determined by the back pressure applied to components connected to the pipe extended to an air electrode, the pressure of air can be controlled within the limited pressure range under the conditions of the operable range below the surge curve.    DETAILED DESCRIPTION - An INDEPENDENT CLAIM is included for method for controlling fuel cell system.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic view of the fuel cell system. (Drawing includes non-English language text)    Fuel cell stack (100)    Compressor (200)    Air supply pipe (300)    Exhaust pipe (320)    Valve (600)    Bypass pipe (700)    Controller (800)