To subject harmful hexavalent chromium to detoxification treatment by introducing a reducing agent into a heat treatment apparatus and treatment facilities to reduce and clean hexavalent chromium to clean the apparatus and the facilities.

A treated matter obtained by the pyrolysis due to a pyrolytic means 20 is introduced into a carbide combustion furnace 36 using feed means 34 and 35 and burnt to be ashed. The combustion furnace 36 is constituted of a rotary furnace 37 and a refractory heat-resistant layer 38 is provided in the rotary furnace 37. A charging jacket 42 is provided on the carbide charging side of the rotary furnace 37 and a combustion burner 43 for burning carbide is also provided. A discharge jacket 45 provided with a refractory heat-resistant layer 44 is provided on the ashed matter discharge side of the rotary furnace 31. The ashed treated matter is recovered in an ash recovery box. The reducing agent is introduced by spraying in such a state that the whole of the apparatus is sucked by an exhaust blower 49 in parts A1-A3 shown by a drawing.