The purpose of the present invention is to provide a rotary hearth furnace exhaust gas duct apparatus which produces reduced iron or granular metal iron by heating a raw material containing a carbonaceous reduction material and an iron oxide-containing substance, which can collect dust while also avoiding problems that accompany increases in exhaust gas amount and temperature, and which also can stably operate for a long period of time at a high thermal efficiency, and to provide a method for operating the rotary hearth furnace exhaust gas duct apparatus. A cooling unit (12) for solidifying metal salt in exhaust gas discharged from a rotary hearth furnace by cooling the exhaust gas, a collision unit (13) for colliding the exhaust gas immediately after cooling to cause the solidified metal salt to fall, and a direction conversion duct (14) for leading the collided exhaust gas in a direction other than the direction in which the metal salt fell are disposed in two stages in the aforementioned order in a rotary hearth furnace exhaust gas duct (8) in the rotary hearth furnace exhaust gas duct apparatus (11) according to the present invention.