To provide a heat treatment method and a heat treating device by which dioxins can be cut at a comparatively low temperature of 300-500°C with the use of a commercial grade powdered decomposer of dioxins.

Ash intended for treatment such as fly ash or incineration ash to which a decomposer of dioxins amounting to 0.2-50 wt.% is added using an adding device 57, is supplied into an externally heated heat treatment furnace 22 consisting of a transversely long cylinder having an internal atmospheric temperature of 300-500°C through an ash feed aperture 32. Further, a feeder 29 made up of a ribbon conveyor arranged in the heat treatment furnace 22 is driven in the repeated forward/backward rotation mode at a cycle of a specified forward rotation time and a specified backward rotation time which is shorter than the former. After that, the ash intended for treatment is moved to a treated ash discharging aperture 33 in the heat treatment furnace 22 while being heated by driving the feeder 29 and then the treated ash is unloaded to the outside from the aperture 33.