PURPOSE: To efficiently obtain dehydrated cake having a low water-retaining ratio, by adding a specified amount of perlite having a particle size of 0.1W2.0mm to sewage sludge contaning a large amount of organic substance, mixing the sludge, flocculating it by the addition of a polymer flocculant, and then dehydrating it.

CONSTITUTION: Sewage sludge containing a large amount of organic substance formed from a sewage-purifying plant is introduced into a mixing cell 1, and perlite having a particle size of 0.1W2.0mm and comprising vitreous volcanic rock such as perlite or obsidian is mixed in the sludge at a ratio of 5W30wt%, preferably 10W20wt%, by solid matter of dried sludge. Thereafter, the sludge is agitated and mixed, flocculated by the addition of a cationic polymer flocculant in a flocculating-mixing cell 2, and then dehydrated by a dehydrator 3 using filter cloth. Since the perlite is added, water ways between sludge particles 7 and the perlite 8 are secured even when the sludge is pressed from the opposite sides of the filter cloth 4, so that water can be easily withdrawn even from the inner layer of the sludge.