A solid oxide fuel cell system (1A, 1B), comprising: a primary fuel supply passage that supplies fuel; a plurality of fuel cell stacks (2, 3) that are provided in line on the primary fuel supply passage and include at least a first fuel cell stack on an upstream side and a second fuel cell stack on a downstream side, and each of which uses a solid oxide fuel cell; a first reformer that is provided on an upstream side from the first fuel cell stack on the primary fuel supply passage, and includes reforming catalytic agents for reforming the fuel by 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 a secondary fuel supply passage connected to the primary fuel supply passage between the first fuel cell stack and the second reformer, characterized in that the solid oxide fuel cell system (1A, 1B) further comprises a fuel pump, an evaporator, a preheater and a combustor, wherein the evaporator, the preheater and the first reformer are provided between the fuel pump and the first fuel cell stack on the primary fuel supply passage in this order from the upstream side, and wherein the preheater heats fuel gas discharged from the evaporator by utilizing heats generated at the combustor, and the combustor generates heats by combusting combustible ingredients contained in the exhaust gas flowing through an off-gas exhaust passage that exhausts exhaust gas of the solid oxide fuel cell system (1A, 1B), the secondary fuel supply passage is diverged from the primary fuel supply passage on an upstream side from the evaporator, the secondary reformer includes - methanation catalytic agents for reforming the fuel by exothermic methanation reactions, and, in addition, - reforming catalytic agents for reforming the fuel by endothermic reforming reactions, and a catalyst core on which reforming catalytic agents and methanation catalytic agents are supported is housed inside the second reformer. a primary fuel supply passage that supplies fuel; a plurality of fuel cell stacks (2, 3) that are provided in line on the primary fuel supply passage and include at least a first fuel cell stack on an upstream side and a second fuel cell stack on a downstream side, and each of which uses a solid oxide fuel cell; a first reformer that is provided on an upstream side from the first fuel cell stack on the primary fuel supply passage, and includes reforming catalytic agents for reforming the fuel by 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 a secondary fuel supply passage connected to the primary fuel supply passage between the first fuel cell stack and the second reformer, characterized in that the solid oxide fuel cell system (1A, 1B) further comprises a fuel pump, an evaporator, a preheater and a combustor, wherein the evaporator, the preheater and the first reformer are provided between the fuel pump and the first fuel cell stack on the primary fuel supply passage in this order from the upstream side, and wherein the preheater heats fuel gas discharged from the evaporator by utilizing heats generated at the combustor, and the combustor generates heats by combusting combustible ingredients contained in the exhaust gas flowing through an off-gas exhaust passage that exhausts exhaust gas of the solid oxide fuel cell system (1A, 1B), the secondary fuel supply passage is diverged from the primary fuel supply passage on an upstream side from the evaporator, the secondary reformer includes - methanation catalytic agents for reforming the fuel by exothermic methanation reactions, and, in addition, - reforming catalytic agents for reforming the fuel by endothermic reforming reactions, and - methanation catalytic agents for reforming the fuel by exothermic methanation reactions, and, in addition, - reforming catalytic agents for reforming the fuel by endothermic reforming reactions, and a catalyst core on which reforming catalytic agents and methanation catalytic agents are supported is housed inside the second reformer.