To decompose a harmful substance such as dioxin with high heat efficiency.

An inner cylinder 3 rotated along with an outer cylinder 2 is set in the outer cylinder 2 which is rotated around its substantially horizontal axis 1 and heated externally, and a feed part 2A for feeding a substance to be treated is disposed on the one-end side of the outer cylinder in its axial direction to constitute the device. Further, an exhaustion part 2B is furnished on the other-end side of the outer cylinder, the inner cylinder is provided with a discharge part 3A on its one-end side and a communicating part 3B communicating with the internal space of the outer cylinder on the other-end side, an outward conveying means for conveying the substance from the one-end side to the other-end side is furnished in the outer cylinder, and an inward conveying means for conveying the substance in the inner cylinder from its one-end side to other-end side is provided in the inner cylinder.