PROBLEM TO BE SOLVED: To extend expiration date of a small membrane module.SOLUTION: The small membrane module 1 includes a membrane storage part 10 storing a hollow fiber membrane A directed to the longitudinal direction, a first nozzle 20 connected to a longitudinal end surface of the membrane storage part 10 and communicated to a primary side of the hollow fiber membrane A, a second nozzle 21 connected to a side surface of the membrane storage part 10 and communicated to a secondary side of the hollow fiber membrane A, and a flexible cap 40 freely mounted to the second nozzle 21. In all the positions of a part mounted with the cap 40 in the nozzle shaft direction, an inner dimension D1 of the cap 40 and an outer dimension D2 of the second nozzle 21 satisfy the relation expressed in the following inequation: 0.75&leD1/D2&le0.95.
