To do thermal decomposition treatment suitable for the kind of waste easily and efficiently by providing a means for setting the inclination of a heating furnace main body variably corresponding to the state (kind, quantity, and others) of waste to be treated.

A heating furnace main body 1 is installed to slant by an angle αto a horizontal plane. The angle α is set up to be appropriately variable corresponding to the state, i.e., the kind, properties, quantity, and others of waste to be treated. In an angle setting operation, one end part in a frame base part 2a which axis-supported the other end part by an axis-support pin 22 is moved vertically by a load jack as shown by an arrow. By changing the angle α of the main body 1 corresponding to the kind and others of water to be treated, a waste heating process and a processing time can be adjusted and set in an appropriate state. In this way, waste can be heat-treated efficiently.