A reformer for a fuel cell system includes a heating source for generating heat by a reaction of a fuel and an oxidant using an oxidizing catalyst, and a reforming reaction part for generating hydrogen by a reforming catalyst reaction. The oxidizing catalyst includes a solid acid, including a strong acid ion and an inorganic oxide, and a platinum-based metal. The reformer for a fuel cell system can start a fuel oxidation catalyst reaction at a low temperature with the heating source having a simplified structure.