PURPOSE: To keep a space between adjacent membrane modules constant even in the change with the lapse of time by providing a projecting part on a membrane module so that the adjacent membrane modules mutually suppresses the generation of deflection.

CONSTITUTION: Plural membrane modules 22 are arranged in parallel in the up-and-down direction at a certain interval in a main body casing 21. A guide groove 23 for guiding the membrane module 22 is provide in the inside face of the main body casing 21. The projecting part 30 to mutually being in contact with each other between the adjacent membrane modules 22 is provided on the front and the back surface of the each membrane module 22.