To achieve a higher filling rate of material to be treated by improving the stirring effect and the heat exchange efficiency of a core tube of a rotary retort kiln.

In this rotary retort kiln, a core tube 2 rotated by a drive motor 17 is installed almost horizontally in a heat reserving part 3 and is filled with material to be treated such as granular solid material to be subjected to heat treatment. In this case, at a part or the whole of the core tube 2, a plurality of tube bodies 22 each with a roughly polygonal cross section are made parallel while being separated mutually to be arranged radially.