An anastomotic instrument includes first and second units. The first unit is attached to a distal end portion of an insert section of an endoscope, and has a self-propellable apparatus and an anvil. The second unit is attached behind the first unit, and has a cartridge and a balloon. The insert section with the anastomotic instrument is inserted into the alimentary canal. In the alimentary canal, fluid is sucked out from space enclosed by the first and second unit through a suction port provided in the cartridge, to bring the space into negative pressure. While the alimentary canal is shrunk in the space, the second unit is shifted relative to the first unit. In a state where the shrunk alimentary canal is caught between the first and second units, a cutter resects living body tissue, and staples are struck to join edges of an incision of the alimentary canal.