The present invention provides a method of dewatering sludge which exhibits an excellent aggregation and dehydration performance on a sludge containing little coarse float and having a high excessive sludge rate. Namely, a method of dewatering sewage sludge, wherein the sewage sludge is a first precipitated sludge formed by a sewage disposal treatment, an excessive sludge or a mixed sludge comprising a first precipitated sludge and an excessive sludge that has a ratio of coarse float (CF) to suspended solids (SS) of 15% by mass or less, characterized by comprising adding a cationic high molecular aggregating agent containing a polymer having 65% by mol or more of a cationic monomer having acryloyl groups in all of the constituting monomer units thereof to the sewage sludge followed by dewatering without resorting to polyamidine.