To provide a humidity adjustment device which is capable of increasing the transferring amount of steam and does not break a membrane due to the deformation by air pressure even if the volume of chamber space is enlarged in dehumidifying or humidifying the indoor humidity, or suppressing the change of humidity by transferring the steam of the closed chamber space such as a box, a pipe, an article housing box and the like.

The humidity adjustment device is prepared by fitting a cap 10 at the uppermost end of a hollow pipe 1 so that it communicates with the inner space of the hollow pipe 1 having a height of 100 m or above and the inner diameter of 30 cm, inserting one end of a communicating tube 11d into each of small apertures 10b installed at the outer periphery of the cap, supporting the other end with an outside fixing plate 12 and a mesh net 13, forming a steam transferring control vessel by installing three leaves of waterproof permeable membranes 11a, 11b, 11c at the inside of each communicating tube 11d wherein the fixing plate 12 and the mesh net 13 are fitted so that they are energized to the direction of the cap 10 with a spring 12c, and arranging a tubular and outer hood 14 to capsulate the cap 10, communicating tube 11d, fixing plate 12 and mesh net 13.