To develop a sludge dewatering agent suitable for a screw press dehydrator by paying attention to the necessity of coagulability with good drainability during the early stage of dewatering as a function of a polymer coagulant required when sludge is dewatered by a screw press dehydrator to introduce the concept of crosslinking into a water-soluble polymer.

In a sludge dewatering method where sewage-mixed raw sludge or sewage-digested sludge receives addition of a high molecular sludge dehydrator and then dewatered by a screw press dehydrator, as the high molecular sludge dehydrator, at least one is selected from among (a) a polyamidine-based water-soluble polymer, (b) a high cationic low molecular weight water-soluble polymer, and (c) a low crosslinkable water-soluble polymer.