PURPOSE: To keep a roller mill always functioning in the finest operating conditions by controlling the moisture of the material in a separator for classification by adjusting the heat retention quantity of a carrier gas introduced into the separator and passing hot gas for final drying of the material through a drying chamber.

CONSTITUTION: The moisture of the material in the separator 3 for classification is controlled by adjusting the heat retention quantity of the carrier gas introduced into the separator. The hot gas for final drying of the material is fed into the drying chamber 9 disposed between the separator 1 for classification and the settling separator 3. As a result, even when the number of circulation times of the material is relatively large, the material to be ground in the roller mill 1 contains the moisture of some extent at all times and consequently, the roller mill 1 may be functioned always under the finest conditions, and eventually the utilization to the maximum possible extent is made possible without requiring the additional water and heat.