A fuel cell system comprising a fuel cell stack and a fuel reformer for reforming a fuel and supplying a reformed fuel to the fuel cell stack, further comprising a total enthalpy heat exchanger for exchanging heat and moisture between a reacted gas and an unreacted gas from the fuel cell stack, a separator for separating moisture from the fuel gas fed from the fuel cell stack or from an oxidant gas, and a mixer for mixing part of the moisture separated by the separator with a medium having a freezing point of 0° C. or less.
