A fuel cell system that effectively processes a flue gas generated from a heat source of a fuel reforming apparatus. The fuel reforming apparatus generates a reforming gas containing hydrogen through a reformation reaction of the fuel from a fuel supply, and a fuel cell main body generates electrical energy through an electrochemical reaction of the reforming gas with an oxidizing agent. The fuel reforming apparatus includes a reforming reaction part and a heat source. The reforming reaction part induces a reforming reaction in the fuel, and the heat source provides heat energy to the reforming reaction part. A flue gas postprocessor induces an oxidation-reduction reaction in a flue gas exhausted by a combustion reaction of the heat source to decrease toxic ingredients, such as CO, hydrocarbons, and NOx, in the flue gas.