To solve problems in an external heating rotary kiln in which the outside of the kiln has to be a combustion chamber made of a refractory material or to be provided with a combustion chamber additionally provided and made of a refractory material, resulting in rendering a plant thereof large, heavy and expensive and an installation space thereof wide.

A conventional continuous rotary kiln has only approximately 10% charging efficiency of objects to be treated. The internal heating rotary kiln is provided with a cylindrical combustion inner casing fixed on a platform at a front part and a rear part thereof inside an excessive space inside a rotating outer casing kiln, and on a position of the combustion inner casing not coming in contact with the objects to be treated and having an approximately 70% diameter of the outer casing kiln. Hoods having an outside surface thereof thermally insulated by ceramic fibers or the like respectively are provided on the front part and the rear part of the outer casing kiln, and the combustion inner casing penetrates into the hoods. For dry distillation, the rotating outer casing kiln and the hoods fixed on the platform have a structure for preventing penetration of air from the outside by sealing packing respectively.