PURPOSE: To process waste water containing hydrophobic organic solvent at a high rate by separating waste water containing hydrophobic organic solvent by differences in specific gravities into water phase and solvent phase and then processing the water phase having diluted solvent concentration by pervaporation membrane method.

CONSTITUTION: While water phase in the lower section in a liquid-liquid separation tank 1 is being preheated, the water is fed into a pervaporation membrane module 5 by means of a feed pump 4 and at the same time the pressure on the passing side of the module 3 is reduced by means of a vacuum pump 9. The water component in the feed water is passed through the membrane due to permselectivity for water vapor at the membrane of the module 5, while the water is vaporized due to the reduced pressure on the passing side so that the water is passed therethrough in the form of vapor. The water vapor having diluted organic solvent on the passing side is condensed at a condenser 10, so that the condensate is discharged from a drain pipe 11 and water vapor not having been condensed is discharged from a purge pipe 12. On the other hand, on a non-passing side, solvent is concentrated and the condensate having concentrated solvent is returned through a return pipe 7, cooled at a cooler 8, and then to a tank 1, so that thereafter said cycle is repeated as one cycle.