Abstract: A fuel cell system comprises: a stack unit having a cathode and an anode and generating electricity by an electrochemical reaction between hydrogen and oxygen; a fuel supply unit for supplying hydrogen to the anode of the stack unit; an air supply unit for supplying air to the cathode of the stack unit; and a gas-liquid separator for removing moisture included in off-gas supplied to a burner of the fuel supply unit from the stack unit. Moisture included in the off-gas is removed by the gas-liquid separator and thus combustion inside the burner is actively performed. As the result, steam is stably supplied to the steam reformer and thus a reforming reaction is smoothly performed, thereby enhancing an entire function of the fuel cell system.