To provide a sludge-dehydration process which efficiently dehydrates sludge, can cope with the seasonal variation in the nature of sludge and hardly dehydratable sludge, and exhibits a good performance for filtration and dehydration.

The method is carried out by using the combination of an anionic water-soluble polymer with a mixture of (A) a cationic water-soluble polymer containing 5 to 100 mol% of one kind selected from specified cationic structural units and 0 to 95 mol% of a nonionic structural unit and (B) an amphoteric water-soluble polymer containing 5 to 70 mol% of one kind selected from the above cationic structural units, 5 to 50 mol% of a specified anionic structural unit and 0 to 90 mol% of a nonionic structural unit.