To provide a fuel cell system preventing overheating of a combustion chamber.

The fuel cell system provided with a fuel cell 1 generating power by using fuel gas and oxidant gas, a combustion chamber 7 mixing and burning exhaust fuel gas and exhaust oxidant gas exhausted from the fuel cell, a cooling means 3 cooling the combustion chamber, and a controller 9 controlling the fuel cell, the combustion chamber and the cooling means, is further provided with a means S22 for estimating a flow volume of the fuel gas flowing into the combustion chamber and a means S22 for estimating a flow volume of the oxidant gas flowing into the combustion chamber. The controller 9 is provided with a calorific value calculating means S22 calculating a calorific volume at the combustion chamber based on the flow volumes of the fuel gas and the oxidant gas estimated, and a cooling time calculating means S12 calculating a cooling time of the combustion chamber based on the calorific volume calculated.