To provide a heating furnace capable of solving various problems generated when a material to be treated with a small grain size is heated.

This heating furnace 10 is constituted so that it heats a material to be treated with a small grain size, is provided with a core tube 12 with a diameter less than that of a furnace body, and heats in the state of the treated material continuously filled therein from the top part of the core tube. In this heating furnace, gas produced by the heating is discharged to the outside through a clearance part between the core tube and the furnace body. By this, since the amount of the gas produced from the treated material is less, the produced amount of flying ash is also reduced. In addition, by adjusting the amount of air led into the furnace body as necessary, such a problem that the treated material is ashed beyond carbonization can be prevented from occurring.