The present disclosure relates to a fuel cell hot box for improving the system efficiency of a fuel cell, wherein all of a fuel cell stack part, an afterburner, a reformer, and an air-heat exchange unit are provided inside a main chamber, fuel may be reformed and preheated using heat of the fuel cell stack part and heat of combustion gas generated by the afterburner, and at the same time, air may be also preheated. Thus, wasting energy can be prevented, the lifetime of the entire system can be increased by cooling the fuel cell stack part and increasing the durability of the fuel cell stack part against thermal stress, and a plurality of fuel cell stack parts share the center chamber, thereby simplifying a configuration of the fuel cell hot box. Further, since the reformer is configured to be vertically slidable, a heat exchange area of the reformer may be controlled in a predetermined manner, and thus a flexible system that may adjust a reforming rate of the fuel according to an operation state of the fuel cell may be configured.