PURPOSE: The titled additive having a viscosity-reducing function and giving a slurry excellent in standing stability and shear stability, which comprises a sulfonated condensate between an arom. compd. and a chlorinated aliph. hydrocarbon.

CONSTITUTION: An additive for a coal/water slurry comprising a sulfonate pred. by sulfonating a condensate obtd. by dehydrochlorinating polycondensation between a mono- or polycyclic arom. compd. such as benzene and a chlorinated aliph. hydrocarbon such as 1,2-dichloroethane in at least an equal mol ratio, or its alkali metal salt, ammonium salt or lower amine salt. 0.01W5wt% said additive is added to a coal/water slurry contg. crushed fine coal of which 50wt% passes through a 200 mesh screen.