A fuel cell system includes a fuel cell for generating electric power by using a fuel, a valve module that controls a flow of a low concentration fuel circulating through the fuel cell, and a flow of a high concentration fuel supplied from a fuel storage unit to the fuel cell, a pump for pumping at least one of the low concentration fuel and the high concentration fuel according to a fuel flow control of the valve module; and a mixer for mixing and supplying the low concentration fuel and the high concentration fuel pumped from the pump to the fuel cell.