An Oxygenator as a medical instrument includes at least one first hollow fiber membrane layer 3A which has a plurality of first hollow fiber membranes 31, is obtained by integrating the plurality of first hollow fiber membranes 31, and forms a shape of a cylindrical body as a whole, and at least one second hollow fiber membrane layer 3A' which is disposed at the outer circumferential side of the first hollow fiber membrane layer 3A in a state of being concentric with the first hollow fiber membrane layer 3A, has a plurality of second hollow fiber membranes 31', is obtained by integrating the plurality of second hollow fiber membranes 31', and forms a shape of a cylindrical body as a whole. Moreover, each of the first hollow fiber membranes 31 is wound around a central axis 300, and each of the second hollow fiber membranes 31' is wound around a central axis 300'. A number of times the second hollow fiber membranes 31' are wound is smaller than a number of times the first hollow fiber membranes 31 are wound.
