To efficiently denitrify nitrogen oxide generated at the time of firing by mixing a dehydrating agent with a sewage sludge to make a powdery material, mixing the powdery material with a finely pulverized material of an expansible shale or an expansible clay and after granulating, firing and foaming in a rotary kiln.

The sewage sludge is mixed with an intermediate process article of cement, which contains quicklime separately taken out from a suspension preheater in a cement firing process, in the ratio by weight of 1/(0.1-0.5) to the sewage sludge to form the powdery material holding water of not troublesome degree for handling. Next, the powdery material is mixed with the expansible shale or the expansible clay and granulated by a granulator such an extruder to form the granulated material 10-90 wt.% in the ratio of the powdery material and containing a large quantity of nitrogen matter. The granulated material is fired at 1050-1300°C for 15-60 min of residence time in the rotary kiln and foamed to efficiently denitrify generated nitrogen oxide to obtain the lightweight aggregate.