The overtube includes a slider within an overtube body, which guides an endoscope and a treatment tool. An endoscope-coupled part and a treatment tool-coupled part are provided inside the slider, and the slider has a dead zone where the forward and backward movement of either the endoscope or the treatment tool does not interlock with the movement of the other and a sensing zone where the forward and backward movement of either the endoscope or the treatment tool interlocks with the movement of the other. The overtube body includes a distal-end-side stopper and a base-end-side stopper that restrict the movable range of the slider. A length L from a coupling position of the endoscope to a distal end position of the overtube body and a length Ls from the coupling position of the endoscope to a distal end position of the endoscope are set to satisfy Ls≧L.
