PURPOSE: To improve combustion rate, decomposition rate of a raw material and safety by introducing the raw material fed from a suspension preheater into a calcining furnace directly joined to the outlet of waste gas of a kiln and feeding both fuel necessary for calcination and all amounts of air bleed of a cooler.

CONSTITUTION: The waste gas of a kiln, air bleed of a cooler, fuel and a raw material are fed to a primary calcining furnace 4 at once. The waste gas of the kiln is introduced into the furnace as a jet through a waste gas introducing port 3 and the raw material is fluidized in the vicinity of a funnellike part 5 and a denitration region is formed by fuel fed through a burner 8. Then the floating layer of the raw material is formed to the upper part thereof and main combustion is performed in this region and a calcination region is formed. The waste gas of the primary calcining furnace is introduced into a secondary calcining furnace 9 while both unburned substance of fuel and the undecomposed raw material remain and receives a mixing action by the drastic change of the cross section of the inlet thereof. Combustion is completed and also decomposition of the raw material is progressed and the waste gas is introduced into a cyclone 12 in the final stage.