PROBLEM TO BE SOLVED: To provide a region ensuring tool which can ensure a field of view of a part to be operated on and a field of operation in a NOTES (Natural Orifice Transluminal Endoscopic Surgery), and maintains an access route to a diseased part in inserting a soft endoscope into an alimentary lumen.SOLUTION: A region ensuring tool 10 to be used in transluminal natural orifice endoscopic surgery includes a cylindrical body part 11 with a hollow endoscope accommodating space in the inside. The body part 11 has, on one end, an insertion port for communicating between an endoscope accommodating space and the outside. A fluid accommodating space capable of accommodating a fluid is arranged in a wall 15 forming the endoscope accommodating space. The body part 11 has such a structure that when the fluid is supplied to the fluid accommodating space, the outer and inner dimensions are increased and a communication path for communicating the endoscope accommodating space to the outside is formed other than an insertion port. When a fluid is supplied to the fluid accommodating space, the outer dimension of the body part 11 is increased. Thus, by arranging the region ensuring tool in a stomach and supplying the fluid to the fluid accommodating space, the stomach can expand.
