A solid oxide fuel cell system includes a primary fuel supply passage for supplying fuel, plural fuel cell stacks each uses a solid oxide fuel cell and that are provided in line on the primary fuel supply passage and includes at least a first fuel cell stack and a second fuel cell stack, a first reformer that is provided on an upstream side from the first fuel cell stack on the primary fuel supply passage and reforms the fuel by utilizing endothermic reforming reactions, a second reformer that is provided between the first fuel cell stack and the second fuel cell stack on the primary fuel supply passage and reforms the fuel by utilizing endothermic reforming reactions and exothermic methanation reactions, and a secondary fuel supply passage connected between the first fuel cell stack and the second reformer.