An overtube to be inserted into a body wall is provided with an endoscope insertion passage and a treatment tool insertion passage, the treatment tool insertion passage is provided parallel to a reference axis of the overtube, and the endoscope insertion passage is provided obliquely to the reference axis. A slider, which is engaged with the endoscope and the treatment tool inserted through the respective insertion passages and causes the endoscope and the treatment tool to interlock with each other and move forward and backward, is arranged inside the overtube so as to be movable in a forward-backward direction of the overtube. An endoscope engagement part engaged with the endoscope in the slider is movable in an upward-downward direction, and moves upward and downward along the position of the oblique endoscope insertion passage together with the movement of the slider in the forward-backward direction.
