Disclosed is a fuel cell system comprising: a fuel cell stack including an anode, a cathode, and an electrolyte membrane disposed therebetween; a fuel tank for supplying hydrogen-including fuel to the anode of the fuel cell stack; and an oxidant supplying unit for adding ozone to oxygen-including air and thereby supplying to the cathode of the fuel cell stack. According to this, ozone is supplied to the cathode of the fuel cell stack thus to accelerate a reaction speed in the fuel cell stack and thereby to obtain a relatively high current density.