An anastomotic instrument (4) includes first and second units (21, 22). The first unit (21) is attached to a distal end portion (5a) of an insert section (5) of an endoscope (3), and has a self-propellable apparatus (30) and an anvil (31). The second unit (22) is attached behind the first unit (21), and has a cartridge (71) and a balloon (72). The insert section (5) with the anastomotic instrument (4) is inserted into the alimentary canal (90). In the alimentary canal (90), fluid is sucked out from space enclosed by the first and second unit (21, 22) through a suction port (83a) provided in the cartridge (71), to bring the space into negative pressure. While the alimentary canal (90) is shrunk in the space, the second unit (22) is shifted relative to the first unit (21). In a state where the shrunk alimentary canal (90) is caught between the first and second units (21, 22), a cutter (80) resects living body tissue (91), and staples (79) are struck to join edges of an incision of the alimentary canal (90).