A high-temperature operating fuel cell system includes: a fuel cell stack that generates electric power through an electrochemical reaction of an oxidant gas and a reformed gas; a combustor that combusts a cathode off-gas and an anode off-gas; a reformer that generates the reformed gas from a raw material by utilizing heat of an exhaust gas generated by the combustor; a first preheater; a second preheater that preheats the oxidant gas through heat exchange with the exhaust gas and supplies the preheated oxidant gas to the cathode of the fuel cell stack; a casing that contains these components; and a first heat insulator that covers at least part of the casing, wherein the first preheater covers the first heat insulator and preheats the oxidant gas by heat transferred from the casing through the first heat insulator before the oxidant gas is supplied to the second preheater.