Fuel cell system including a fuel cell and a controller for controlling water vapor amount or average flow rate of a fuel gas

A fuel cell system having a fuel cell operated under non-humidified conditions that includes a polymer electrolyte membrane sandwiched between an anode and a cathode, a fuel gas channel facing the anode to supply it with fuel gas, an oxidant gas channel facing the cathode to supply it with oxidant gas, and a flow direction of the fuel gas and the oxidant gas are opposite. The fuel cell system may control a water vapor amount at an outlet of the fuel gas channel based on a value that is set based on a relationship between a voltage of the fuel cell and the water vapor amount. The fuel cell system may control an average flow rate of the fuel gas in the fuel gas channel based on a value that is set based on a relationship between a voltage of the fuel cell and the average flow rate.