A treatment instrument for medical use includes: an outer tube having a flexible tube at a distal portion thereof an image information acquisition unit disposed inside the outer tube and configured to be inserted into and drawn out of the flexible tube and a fluid conveying tube which is disposed inside the outer tube juxtaposedly with the image information acquisition unit and is insertable into and drawn out of the flexible tube in a selective manner in relation to the image information acquisition unit. The outer tube is greater than the flexible tube in diameter and is decreased in diameter, at a connection part for connection with the flexible tube, down to the diameter of the flexible tube.
