PURPOSE: To improve the strength, yield strength or the like of solidified cement products by adding water to the material to be ground in a roller mill at the time of finish-grinding the cement in a roller mill plant.

CONSTITUTION: Water is added to the material to be ground in the roller mill at the time of finish-grinding the cement by supplying cement material consisting of cement clinker, gypsum or the like to the roller mill plant. The addition of the water may be executed before the cement material is introduced into the roller mill or the water may be added to the cement material by supplying a hydraulic cement component to the roller mill. The water may otherwise be added to the cement material by water spraying at the several points of the roller mill. Further, the water addition to the cement material is preferably controlled as a function of a product parameter number. As a result, prehydration of the cement is induced by adding the water to the cement material, by which the amt. of the water to be added at the time of cement production may be decreased.