PURPOSE: To miniatuarize the apparatus and to reduce treating energy, by arranging the first step treatment vessel performing the sludge freezing melting, the vessel separating treated sludge into supernatant liquid and concentrated sludge and the second step freezing melting treatment vessel of the concentrated sludge.

CONSTITUTION: The first step treatment vessel 1 performing freezing melting treatment of the difficult filterable sludge A and the sludge storage vessel 21 separating the sludge treated in the vessel 1 into the supernatant liquid A2 and the concentrated sludge A1, are arranged and the discharge hole 21a discharging said liquid A2 outside, is provided on the vessel 21. Next, the apparatus is constructed so as to treat said sludge A1 by the second step treatment vessel 31 performing the freezing melting treatment of only said sludge A1 again and making the cake B by treating said sludge A1 by the dehydrator 4. By this apparatus, said sludge A1 is made a half compared with the usual method and freezing melting energy can be reduced and also, volume of the treating vessel is reduced by half.