At a distal portion of a shaft of a guide portion included in a medical device, a first portion is configured to be capable of coming into contact with an inner wall of a living organ. A second portion is disposed on a proximal side of the shaft with respect to the first portion. The second position is configured to be capable of coming into contact with the inner wall of the living organ at a position different from that of the first portion on a transverse cross section of the inner wall of the living organ. A treatment portion is movable in a lumen of the shaft in a state where at least a part of the first portion and at least a part of the second portion come into contact with the inner wall of the living organ.