PURPOSE: To efficiently dehydrate sludge with reduced power consumption, by adjusting the conductivity of raw sludge to a low value to modify the same before subjecting said sludge to electroosmosis dehydration.

CONSTITUTION: Raw sludge is charged into a washing tank and mixed with washing water such as tap water sufficiently low in the concn. of an electrolyte under stirring. Sludge interstitial water high in conductivity contained in raw sludge is thinned in the concn. of the electrolyte thereof by this operation and conditioned so as to lower the conductivity thereof and excessive washing waste water is further separated from sludge and drained. Sludge adjusted to low conductivity by this method is sent into an electroosmotic dehydration process and subjected to dehydration treatment by an electroosmotic dehydrator to be recovered as a low moisture cake. Furthermore, pre-concn. due to the addition of a flocculant and pre-dehydration may be performed before sludge adjusted to low conductivity is sent to the electroosmotic dehydration process.