A sludge dehydrating agent comprising a cationic polymer which has: a constitutional unit a derived from a quaternary ammonium salt having a specific structure; and at least one constitutional unit selected from among a constitutional unit b derived from a quaternary ammonium salt having a specific structure and a constitutional unit c derived from a tertiary amine salt having a specific structure, wherein the intrinsic viscosity of the cationic polymer at 30° C. in a 1 mol/L sodium nitrate aqueous solution is 0.5-4.0 dL/g.