PURPOSE: To improve a thermal efficiency, to lower a temperature of an outer surface, and to effectively prevent a deformation of a furnace shell steel plate by providing an air layer between the plate and an insulating wall in a furnace.

CONSTITUTION: The insulating structure for a heating furnace comprises a furnace shell steel plate 1 for constituting a body frame, and an insulating wall 2 in the furnace, made of ceramic fiber. A steel plate bent in an uneven state, so-called a deck plate 10 is interposed between the plate 1 and the wall 2, and the outer surface of the plate 10 is mounted on the inner surface of the plate 1 by a method such as a plug welding, etc. Then, an air layer 11 formed in thickness corresponding to a size between the inner surface and the outer surface of the plate 10, is provided between the plate 1 and the wall 2. The layer 11 is operated as a function of a heat insulating layer between the plate 1 and the wall 2 of different temperatures from each other. Thus, the layer 11 prevents transfer of heat generated in the furnace to the plate 1 through the wall 2.