PURPOSE: To prevent a liquid film from forming on a cooling plate, to increase the heat exchange efficiency between vapor and the cooling plate and to obtain efficiently the permeated liquid by making the surface which is opposed to a porous film of the cooling plate of a thermo-perevaporation apparatus water repellent.

CONSTITUTION: In a perevaporation apparatus constituted so that a flow path 2 of raw liquid and a flow path 3 of permeated liquid are partitioned by the porous films 1 and the vapor permeated through the porous films 1 is condensed on a cooling plate 4, the surface of the cooling plate 4 opposing to the porous films 1 is constituted of a water repellent layer 4a by means of a method coating the fluororesin thereon. The vapor condensed on the surface of the cooling plate 4 becomes liquid droplet 6 shape and the heat exchange between vapor and the cooling plate is smoothly performed.