The fuel cell system 1 has a reformer 2 and a fuel cell 3. The reformer 2 has a reforming reaction channel 21 that generates a hydrogen-containing reformed gas Ga and a heat exchange channel 22 for heating. The fuel cell 3 has an anode channel 32 to which the hydrogen-containing reformed gas Ga is supplied, a cathode channel 33 to which an oxygen-containing gas Gc is supplied, and an electrolyte 31 formed between them. The electrolyte 31 is a laminate of a hydrogen-separating metal layer 311 and a proton conductor layer 312. The fuel cell system 1 has a cathode offgas line 46 for feeding the cathode offgas Oc discharged from the cathode channel 33 to the reforming reaction channel 21.