MEMBRANE SEPARATOR

PROBLEM TO BE SOLVED: To provide a membrane separator that can reduce a diffusion amount and improve uniformity of diffusion.SOLUTION: A membrane separator is a membrane separator S which obtains a treated liquid by subjecting a liquid to be treated to a filtration treatment, and includes: a membrane element 4 which is provided with a plane film type filter membrane 4r for filtrating the liquid to be treated so that a pair of extension surfaces are opposed to each other to form an outline, and obtains the treated liquid by subjecting the liquid to be treated to filtration through the filter membrane 4; a membrane module 5 in which a plurality of membrane elements 4 are provided side by side horizontally with inter-membrane flow passages 5s therebetween so that extension surfaces of the membrane elements 4 are in a perpendicular direction; a membrane separator tank 3 in which the membrane module 5 is arranged; and diffusion pipes 8a having diffusion holes 8a1 formed to diffuse air to the liquid to be treated in the inter-membrane flow passages 5s, axes of the diffusion pipes 8 being arranged crossing extending directions of the film elements 4 in an upper plane view.