PURPOSE: To provide the titled compact apparatus easy to operate, constituted so as to finely pulvelize an alkali stock material by a pulverizer while mixing the pulverized stock material with carbon dioxide-containing prepared water in a thickener to obtain hard water.

CONSTITUTION: Required carbon dioxide 2 is added to the prepared water 1 obtained in a seawater desalting apparatus and dissolved therein in a line mixer 3 while the make-up water is sent to the slurry mixing tank 23 of a reactivator type thickener 22. On the other hand, an alkali stock material such as dolomite is introduced into a wet tower mill 20 from a silo 6 and a part of the prepared water 1 is added to said mill 20 to finely pulverize the alkali stock material while the prepared slurry is sent into the above mentioned mixing tank 23 through a slurry tank 21. Herein, both of them are mixed under stirring by a stirring blade 25 to obtain hard water with predetermined M-alkalinity. The coarse particles in this hard water is separated by a precipitation and separtion tank 26 while treated hard water is recirculated to the mixing tank 23 by a pump 29 and hard water of an upper layer is flooded to be sent to a filter tank 27 where contained SS is removed to obtain purified hard water. This hard water is stored in a tank 9 and taken out by a pump 15 if necessary and a predetermined amount of chlorine gas 16 and sodium hydroxide 17 are added to hard water to obtain drinking water 14.