The present invention provides a fuel cell system that controls a pressure for a fuel gas supplied to a fuel cell so that the anode-cathode differential pressure between an anode and a cathode of a fuel cell is maintained within a predetermined range, in order to provide a desired generation amount while reducing the amount of fuel gas discharged to the exterior of the system even if it is difficult to provide an appropriate fuel gas supply amount for a fuel cell load (the generation requirement on the cell). If the system determines that the amount of fuel gas supplied to the fuel cell is less than an appropriate required gas amount for the load (generation requirement) (step S9: NO), for example, if the concentration of nitrogen in the fuel gas reaches a predetermined value or greater, the open and close state of shut-off valves H3, H3A is switched to increase the pressure for the fuel gas supplied to the fuel cell 20 (step S11).