A medical retainer is provided capable of independently controlling supplying air and water to a hollow organ like a stomach and a lumen of a small intestine. The medical retainer for sealing a fluid, the retainer being subject to be retained in a gastrointestinal tract of a patient, the medical retainer includes: a sealing section for sealing the fluid, the sealing section making contact with a wall of the gastrointestinal tract； a main body having passageways； and directional valves disposed in the passageways for allowing one of upward flow and downward flow of the fluid relative to the position of the retainer.
