To reduce the dispersion of heat by a method wherein a second rotary tube, whose one end side in a longitudinal direction is arranged at the other end side of a first rotary tube in a main body and which effects second heat treatment of a work to be treated through heat treatment, supplied from the lengthwise one end side, is provided.

A first rotary tube 32 is received at one end side 31a in a longitudinal direction in a furnace main body 31 while a second rotary tube 33 is received at the other end side 31c in the furnace main body 31. The conical rotary tube 37 of the first rotary tube 32 is heated by an electric heater 34 and also the heat of flame, produced by a gas burner 35 and supplied through a waste heat supplying passage 36. When the conical rotary tube 37 under heated condition is rotated and slurry type ceramic material G is supplied into the conical rotary tube 37, the slurry type ceramic material G is brought into contact with a plurality of heat resistant rolling bodies 41 under heated condition whereby the material G is dried. Next, the ceramic material G, discharged out of the first rotary tube 32 and supplied into the second rotary tube 33, is heated by the heat of flame, produced from a gas burner 35, whereby the material is calcined.