PURPOSE: To efficiently separate a heavy metal from industrial waste water, by adding borax or boric acid to a complex compound of polyvinyl alcohol and metal hydroxide to insolubilize said complex compound and forming heavy metal hydroxide into a particle excellent in filterability.

CONSTITUTION: 3W10% of polyvinyl alcohol is added to metal hydroxide-containing sludge as a solid on the basis of the dry solid of heavy metal hydroxide and the resulting mixture is well stirred to form a metal complex compound through complexing reaction. Borax, boric acid, sodium sulfate or potassium sulfate is added to this metal complex compound-containing sludge alone or in combination in an amount of 3W15% by wt. of the metal hydroxide dry solid not only to make the metal complex compound and unreacted polyvinyl alcohol insoluble in water but also to bring water-containing sludge to a sponge state. This treated sludge is dehydrated by a filter press to obtain a dehydrated cake with water content of 60W70% having a reduced cake volume.