Abstract: An object is to provide a fuel cell system having a simple structure, which is capable of supplying gas appropriately through an ejector in accordance with the load of a fuel cell. The fuel cell system (1) uses an ejector (24) disposed in a gas supply system (4) to combine a new gas to be supplied to the fuel cell (2) with an off-gas discharged from the fuel cell (2) and supply the fuel cell (2) with the resulting combined gas. The ejector (24) includes a nozzle (46) for ejecting the new gas and generating a negative pressure for aspirating the off-gas, and a flow rate control mechanism (47) for controlling the flow rate of the new gas which passes through the nozzle (46). A first flow path (81) for leading the off-gas to the flow rate control mechanism (47) is provided in the gas supply system (4), and the flow rate control mechanism (47) controls the flow rate of the new gas in accordance with the pressure of the off-gas led from the first flow path (81).