To develop a sludge dehydrating agent not increased in its addition amount even if a crosslinkable water soluble polymer is used and lowering water content of a dehydrated cake, for the purpose of solving the problem that excellent points are developed from aspects of the lowering in the water content of the dehydrated cake or the like, when the crosslinkable water soluble polymer is used as the sludge dehydrating agent but an addition amount of the crosslinkable water soluble polymer must be increased in order to develop a dehydration effect of the sludge, and the dehydration cost of the sludge is increased.

The dehydration treatment of the sludge is performed by adding an ionic polymeric compound having a crosslinking structural unit represented by formula (1) to the sludge.