which the exhaust gas from a combustion furnace 3 is introduced into the incipient calcination furnace the manner of supplying combustion air through the combustion air duct 13 the flow of the combustion gas in the incipient calcination furnace 2 and the manner of discharging the combustion gas from the incipient combustion furnace 2 referring to fig3 to 5 the description is directed to the details of the first embodiment to explain its features in construction the second lowest dust separator c3 of the preheating system which supplies a preheating raw material to an incipient calcination furnace 2 includes a fine dust separating means consisting of for example a cyclone 21 having an opening 22 in the side wall thereof and a coarse dust separating means consisting of a pocket 20 fixed to the side wall of the cyclone 21 in communication with the opening 22 and having a coarse dust discharge port 24 at the bottom of a lower pocket portion 23 of an inverted truncated cone shape the dust discharge port 24 of the pocket portion 20 is connected through a coarse dust chute 14a to a combustion chamber 2a at a position close to an air feed port 2e and immediately above a fuel feeder 6a in such a manner as to direct the coarse dust toward a combustion zone 16 on the other hand a fine dust discharge port 26 of the cyclone 21 is connected to a fine dust chute 14b opening into the combustion chamber 2a at a position which is shifted in a circumferential direction to be remote from.