A fuel cell system and related method are disclosed having a system controller 37 that executes enthalpy calculation based on status signals, related to anode off-gas and cathode off-gas, detected by a temperature detector 29, a pressure detector 30 and a humidity detector 31 for thereby predicting a combustion temperature of a combustor 7 during purging. In the presence of a predicted combustion temperature exceeding an upper limit, cathode off-gas is increased in volume to limit a combustion temperature of the combustor 7 to a value below the upper limit value.