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 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. A contact surface of the overtube body with a body wall is constituted of a wall surface member that serves as a rotation-preventing wall surface for preventing rotation in a rotational direction with an axial direction of the overtube body as a center.
