PURPOSE: To selectively permeate and recover only water containing no oil or solvent immiscible with oil, by carrying out permeating separation limiting the composition and temperature of surface active agent and treating liquid containing liquid and oil by porous membrane obtained from the resin(mixture) specified with the critical surface tension.

CONSTITUTION: Permeating separation of treating liquid containng oil, is carried out under the following condition, using porous membrane obtained from the resin having more than 35 dyn/cm of critical surface tension or these resin mixture. That is, Z satisfied Z>0 when 0.02≤α<0.5 and Z>1.78×10-3{α-(0.5+/5Tb)} and Z>0 when 0.5≤α≤0.8. But, α= X/(X+Y), X; volume partial coefficient of oil in treating liquid, Y; volume partial coefficient of liquid except oil, Z; quantity (mol/l) of surface active agent in treating liquid, Tb; boiling point (1 atm.) of liquid to be separated, T; treating temperature (90°C≥T≤35°C)