A furnace sidewall structure of a rotary hearth furnace for making direct reduced iron which is agglomerated iron ore or waste iron oxide containing a reducing agent on a rotary hearth by heating or for heating at least one steel piece, wherein the rotary hearth furnace includes an outer furnace sidewall and an inner furnace sidewall, wherein at least one of the outer furnace sidewall and the inner furnace sidewall is polygonal. The furnace sidewall structure can be prepared with lower manufacturing costs relative to conventional circular profile rotary hearth furnace with respect to the furnace sidewall. The production thereof is easy with respect to manufacturing and with respect to the fact that the dimensional accuracy of the manufactured furnace sidewall can be easily adjusted. Also, there is associated a shorter installation/construction work period and a reduction in the installation/construction cost.