A high-temperature operating fuel cell system comprising: a fuel cell stack that includes a cathode and an anode and generates electric power through an electrochemical reaction of an oxidant gas supplied to the cathode and a reformed gas supplied to the anode; a combustor that combusts a cathode off-gas discharged from the cathode of the fuel cell stack and an anode off-gas discharged from the anode; a reformer that generates the reformed gas from a raw material supplied, the reformed gas being generated through a reforming reaction 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 whose heat has been partially used for the reforming reaction in the reformer and supplies the preheated oxidant gas to the cathode of the fuel cell stack; a casing that contains the fuel cell stack, the combustor, the reformer, and the second preheater; and a first heat insulator that covers at least part of an outer circumference of the casing, wherein the first preheater covers an outer circumference of the first heat insulator and preheats the oxidant gas by heat transferred from inside of the casing through the first heat insulator before the oxidant gas is supplied to the second preheater, the high-temperature operating fuel cell system further comprises: a fuel cell stack container that contains the fuel cell stack and allows the cathode off-gas discharged from the cathode of the fuel cell stack to flow inside the fuel cell stack container and to be introduced into the combustor; and characterized by a third preheater that is disposed on an inner wall of the fuel cell stack container, preheats, by heat of the cathode off-gas flowing through the fuel cell stack container, the oxidant gas preheated in the second preheater before the oxidant gas is supplied to the fuel cell stack. a fuel cell stack that includes a cathode and an anode and generates electric power through an electrochemical reaction of an oxidant gas supplied to the cathode and a reformed gas supplied to the anode; a combustor that combusts a cathode off-gas discharged from the cathode of the fuel cell stack and an anode off-gas discharged from the anode; a reformer that generates the reformed gas from a raw material supplied, the reformed gas being generated through a reforming reaction 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 whose heat has been partially used for the reforming reaction in the reformer and supplies the preheated oxidant gas to the cathode of the fuel cell stack; a casing that contains the fuel cell stack, the combustor, the reformer, and the second preheater; and a first heat insulator that covers at least part of an outer circumference of the casing, wherein the first preheater covers an outer circumference of the first heat insulator and preheats the oxidant gas by heat transferred from inside of the casing through the first heat insulator before the oxidant gas is supplied to the second preheater, the high-temperature operating fuel cell system further comprises: a fuel cell stack container that contains the fuel cell stack and allows the cathode off-gas discharged from the cathode of the fuel cell stack to flow inside the fuel cell stack container and to be introduced into the combustor; and characterized by a third preheater that is disposed on an inner wall of the fuel cell stack container, preheats, by heat of the cathode off-gas flowing through the fuel cell stack container, the oxidant gas preheated in the second preheater before the oxidant gas is supplied to the fuel cell stack. a fuel cell stack container that contains the fuel cell stack and allows the cathode off-gas discharged from the cathode of the fuel cell stack to flow inside the fuel cell stack container and to be introduced into the combustor; and characterized by a third preheater that is disposed on an inner wall of the fuel cell stack container, preheats, by heat of the cathode off-gas flowing through the fuel cell stack container, the oxidant gas preheated in the second preheater before the oxidant gas is supplied to the fuel cell stack.