A fuel cell hot box for improving the system efficiency of a fuel cell. Fuel cell stack parts, an after burner, a reformer, an air pre-heating zone, and a fuel-heat exchanger are provided in a housing allowing heat of the fuel cell stack parts and heat of combustion gas generated in the after burner to be used for reforming, preheating fuel and preheating air at the same time to avoid wasting energy. The fuel cell stack parts under thermal stress can be cooled to improve durability of the stack parts to increase a lifetime of a total system, and the stack parts can share the central chamber part to simplify a structure of the fuel cell hot box. In addition, the reformer includes an opening and closing unit to properly distribute the high-temperature combustion gas so that a reforming ratio is adjustable according to an operating condition of the fuel.