PURPOSE: To make it possible to prepare an aggregate material from dust always availably in rapid temp. control, by a method wherein a bypass duct is connected to the throw-in chamber of a kiln to spray cooling water into the kiln when the temp. therein is raised and sintering exhaust gas is returned to a preheater.

CONSTITUTION: In an apparatus for preparing an aggregate material from dust, a bypass duct is connected to the throw-in chamber of a kiln and a humidifying machine is attached to the throw-in chamber of the kiln or the bypass duct. When the temp. in the rotary kiln is raised by combustion of the uncombustion component in dust pellets, cooling water is sprayed from said humidifying machine. In addition, the baking exhaust gas from the rotary kiln is passed through the throw-in chamber of the kiln and the part thereof is introduced into a preheater from the bypass duct and the residual part thereof is introduced into a dust collector along with the exhaust gas of the preheater. By this mechanism, regardless of the content of the uncombustion component in dust, the aggregate material can be prepared by treating dust.