An anastomosis instrument that includes: a puncture needle that moves reciprocally inside a channel that is provided in an insertion portion that is inserted into a body cavity, and that protrudes from a distal end of the insertion portion so as to puncture a living body tissue and a space retaining device that is positioned in front of a distal end of the puncture needle, and that provides a predetermined clearance between the living body tissue that is to be punctured that is loaded in the distal end of the insertion portion, and the living body tissue that is not to be punctured that is positioned further from the puncture needle than the living body tissue that is to be punctured.
