PURPOSE: To concentrate raw milk inexpensively and easily, by bringing raw milk heated to a given temperature into contact with a porous membrane of hydrophobic polymer through which steam is passed but water is not.

CONSTITUTION: The membrane pipe 2 consisting of a porous membrane of hydrophobic polymer is set on the same shaft in the outer pipe 1, and the heat transfer pipe 9 is laid in the membrane pipe. The porous membrane of hydrophobic polymer is obtained by making a great number of holes with 0.05W50μm, preferably 0.1W10μm in diameter, and it is permeable to steam but impervious to water. Raw milk is heated to a temperature not to cause denaturation but to generate steam, e.g. about 40°C, and it is passed through the passageway 3 between the outer pipe 1 and the membrane pipe 2. Steam that passed through the membrane pipe 2 and diffused into the interior is cooled and condensed by the heat transfer pipe 9 into water, and exhausted from the outlet pipe 13.