Fuel cell system

A fuel cell system including a fuel cell which generates electric energy through chemical reaction between hydrogen and oxygen, comprising:

• an off-gas circulation path for recirculating an off-gas, including unreacted hydrogen, emitted from said fuel cell, to said fuel cell;
• outside air temperature detecting means for detecting an outside air temperature in the vicinity of said fuel cell;
• outside air temperature estimating means for, after said fuel cell enters a deactivated state, estimating a transition of outside air temperature on the basis of a variation of said outside air temperature detected by said outside air temperature detecting means;
• freezing decision means for making a decision as to whether or not the estimated outside air temperature obtained by said outside air temperature estimating means falls below a predetermined freezing temperature at which moisture freezes;
• and
• freezing prevention processing means for, when said freezing decision means makes a decision that the estimated outside air temperature falls below said predetermined freezing temperature, removing moisture contained in the off-gas for preventing the moisture from freezing in the interior of said fuel cell.