An overtube includes a slider within an overtube body, which guides an endoscope and a treatment tool into a body cavity. An endoscope-coupled part and a treatment tool-coupled part are provided in 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. A fluid-supplying connector that is provided on a base end surface of the overtube body, and has an axial direction that is made parallel to a longitudinal axis of the overtube body, and is detachably connected to an air supply tube for sending fluid into the body cavity through an internal space of the overtube body.
