A fuel cell system and a reformer for a fuel cell system prevents backfire and improves efficiency of heat transfer. The fuel cell system includes a reformer generating hydrogen gas from fuel including hydrogen by a catalytic chemical reaction using heat energy, and at least one electricity generating unit generating electrical energy by an electrochemical reaction between the hydrogen gas and oxygen. The reformer includes a case, a heat source, and a reforming reaction part. The case forms an external shape. The heat source is disposed in the case to generate heat energy by an oxidation reaction between fuel and a catalyst, and includes a mesh, an oxidation catalyst layer formed on a surface of the mesh, and at least one fuel injection nozzle supplying the fuel to the oxidation catalyst layer. The reforming reaction part is disposed in the case to generate hydrogen gas from fuel using the heat energy generated from the heat source.