The 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 side hole for sending a fluid into a body cavity is formed in a side wall of the overtube body, and a side wall is configured to be openable and closable by the forward and backward movement of the slider.
