To substantially and continuously move an object to be treated to a drain port side without making a counterflow to a feeding port side by making an opening diameter of the feeding port smaller than that of the drain port and making a center axis of a cylindrical tube horizontal.

A reactor core tube 1 for a rotary kiln (a reactor core tube) is the one made of ceramics having both opened ends and a shape of both end portions 4 is scolded compared with a mother tube portion. The reactor core tube 1 is mounted to a rotary kiln body at an optional supporting portion. When an object to be treated is fed from a feeding port 2 of the reactor core tube 1 by rotating around a center axis of the reactor core tube 1, the object to be treated is moved to a drain port 3 side while rolling in the reactor core tube 1. Since an opening diameter of the feeding port 2 of the reactor core tube 1 is smaller than that of the drain port 3, the object to be treated substantially and continuously moves to the drain pot 3 side without making a counterflow to the feeding port 2 side even if a center axis of the reactor core tube 1 is made horizontal.