PURPOSE: To easily remove coarse particles and, at the same time, to obtain the title slurry having a closest packing particle size distribution, by treating a low concn. coal-water slurry in a ball mill to obtain a dehydrated cake and feeding the dehydrated cake together with water and a dispersant into another ball mill.

CONSTITUTION: Crushed coal 12 and water 13 is fed into a first ball mill 11 and then wet pulverized until the coal concn. and the particle size are 45W55wt% and 74μm, respectively, thereby preparing a low-concn. coal-water slurry 14. The slurry 14 is sieved while diluting it with water 15, thereby preparing a coarse particle slurry 17 contg. coarse particles of 150μm or more and having a coal concn. of 30wt% and a dilute slurry 18 having a coarse particle content of about 1W2% or less and a coal concn. of about 30wt% or less. The slurry 17 is returned into the first ball mill 11 through a pump 19, where it is again ground. On the other hand, the slurry 18 is fed into a dehydrator 21 through a pump 20, where it is dehydrated to prepare a dehydrated cake 22. The dehydrated cake 22 is then fed into a kneader 23 for kneading. The kneading product is fed together with water 24 and a dispersant 25 into a second ball mill 26, follwed by wet grinding, thereby preparing a high-concn. coal-water slurry 27.