An Oxygenator as a medical instrument includes at least one first hollow fiber membrane layer comprised of a plurality of integrated first hollow fiber membranes, and forms a shape of a cylindrical body as a whole, and at least one second hollow fiber membrane layer disposed at the outer circumferential side of the first hollow fiber membrane layer in a state of being concentric with the first hollow fiber membrane layer, has a plurality of integrated second hollow fiber membranes, and forms a shape of a cylindrical body as a whole. Moreover, each of the first hollow fiber membranes is wound around a central axis, and each of the second hollow fiber membranes is wound around a central axis. The number of times the second hollow fiber membranes are wound is smaller than the number of times the first hollow fiber membranes are wound.
