Abstract: A medical device for the transfer of substances, includes a cylindrical housing and, a hollow fiber bundle of a plurality of hollow fiber membranes for transfer of the substances which bundle is inserted within the housing. A first substance transfer chamber is defined by the outer surfaces of the hollow fiber membranes and the inner wall surfaces of the housing, and a first substance transferring inlet and an outlet both communicate with the first substance transfer chamber. Partitions are arranged to support fixedly the opposite ends of the hollow fiber membranes in position, and separate the ends from the substance transfer chamber. A second substance transferring fluid inlet and an outlet communicate with the interior spaces of the hollow fiber membranes, which inlet and outlet are formed together with flow path forming members attached to the opposite ends of the housing, wherein each flow path forming member is provided with an annular protuberance.