A medical device for endoscopically conducting negative pressure therapy on a bursiform cavity having an opening in a body, the medical device comprising: a longitudinal axis member which has a lumen extending along a longitudinal axis and the longitudinal axis member being capable of being inserted into the body; an expansion part which is provided at a distal end of the longitudinal axis member, the expansion part having a plurality of wire members; a suction member which is capable of being inserted into the bursiform cavity, the suction member being formed by a contact surface for coming into close contact with an inner wall of the bursiform cavity when the suction member is inserted into the bursiform cavity and a porous member having a plurality of vacancies, the suction member covering an outer surface of the expansion part; a suction source which is provided at a proximal end of the longitudinal axis member and connected to the lumen and configured to suction the inner wall of the bursiform cavity via the suction member; and a closing member which is provided at an axial end of the suction member so as to airtightly seal the opening of the bursiform cavity, characterized in that the medical device comprises a storage sheath and in that each of the plurality of wire members of the expansion part is configured to be deformed in a straight shape to be stored in the storage sheath when introduced into the bursiform cavity, and when the suction member is advanced straight toward an opening of the bursiform cavity to a position where the suction member contacts with the bottom of the bursiform cavity, the storage sheath is configured to be retracted to enlarge the expansion part such that the plurality of wire members are deformed outward into their natural state in a radial direction of the longitudinal axis member so as to follow the inner wall of the bursiform cavity. a longitudinal axis member which has a lumen extending along a longitudinal axis and the longitudinal axis member being capable of being inserted into the body; an expansion part which is provided at a distal end of the longitudinal axis member, the expansion part having a plurality of wire members; a suction member which is capable of being inserted into the bursiform cavity, the suction member being formed by a contact surface for coming into close contact with an inner wall of the bursiform cavity when the suction member is inserted into the bursiform cavity and a porous member having a plurality of vacancies, the suction member covering an outer surface of the expansion part; a suction source which is provided at a proximal end of the longitudinal axis member and connected to the lumen and configured to suction the inner wall of the bursiform cavity via the suction member; and a closing member which is provided at an axial end of the suction member so as to airtightly seal the opening of the bursiform cavity, characterized in that the medical device comprises a storage sheath and in that each of the plurality of wire members of the expansion part is configured to be deformed in a straight shape to be stored in the storage sheath when introduced into the bursiform cavity, and when the suction member is advanced straight toward an opening of the bursiform cavity to a position where the suction member contacts with the bottom of the bursiform cavity, the storage sheath is configured to be retracted to enlarge the expansion part such that the plurality of wire members are deformed outward into their natural state in a radial direction of the longitudinal axis member so as to follow the inner wall of the bursiform cavity.