A method for concentrating a water-soluble organic material wherein a mixture of the water-soluble organic material with water is distilled in a distillation column (11), and the fraction from the top (11a) of the distillation column (11) is separated by the use of a membrane separator (14) into a permeated vapor (F.sub.2) and a non-permeated vapor (F.sub.3), which comprises once condensing the fraction to form a condensate, heating the condensate in an evaporator (13) to generate a vapor (F.sub.1) having a pressure higher than the operation pressure for the distillation column (11), and introducing the vapor (F.sub.1) to the membrane separator (14), thereby separating water from the mixture.