To inexpensively reduce NOx in an exhaust gas generated in a rotary kiln.

A material is charged from a material supply device 20 into a kiln of the rotary kiln 10, and an additive agent for denitration is sprayed to a part of an adequate temperature zone AP having high NOx reduction effect in the kiln by a denitration additive agent supply device through first and second piping portions 70, 80 connected through a rotary joint 90. The first piping portion 70 is disposed coaxially with a kiln rotating shaft P, and the second piping portion 80 is extended to a kiln outer peripheral side from a kiln rotating shaft P side on a kiln end 12 side, is fixed along the longitudinal direction of the rotary kiln 10 from a kiln outer peripheral side, and is penetratingly disposed in the kiln from the kiln outer peripheral side in the part of the adequate temperature zone AP.