A fuel cell system according to exemplary embodiments of the present invention includes a fuel cell stack that generates electrical energy through the electrochemical reaction of fuel and oxidant, a fuel supply unit for supplying the fuel to the fuel cell stack, and an oxidant supply unit for supplying the oxidant to the fuel cell stack. The fuel supply unit includes a fuel permeation membrane between a space where fuel recovered from the fuel cell stack is positioned and a space where stored fuel is positioned. The fuel cell system enables easy control of the concentration of fuel supplied to the fuel cell stack.