A CO-shift device includes a main body having therein a space in which a CO-shift catalyst is accommodated, the space being divided into an inner space and an outer space surrounding the inner space; an inlet portion formed at one end portion of the inner space, the inlet portion being supplied with a reformed gas such that the reformed gas flows through the inner space. An outlet portion is formed at one end portion of the outer space and a redirecting portion is provided between the other end portion of the inner space and the other end portion of the redirecting portion, thereby reversing the reformed gas flown into the other end of the inner space in order that the resulting reformed gas passes through the outer space to be exhausted from the outlet portion, the reformed gas being shifted to reduce CO by the CO-shift catalyst during its movement through the inner and outer spaces.