An overtube, which is punctured into a body wall and guides an endoscope and a treatment tool into a body cavity, includes an interlocking member that interlocks the forward and backward movements of the endoscope and the treatment tool with each other. The interlocking member is constituted of a first sleeve coupled to the endoscope, a second sleeve coupled to the treatment tool, and a coupling member having an elastic body, which couples the first sleeve and the second sleeve together. Accordingly, in a case where the stretching of the coupling member is small, the endoscope interlocks with the forward and backward movement of the treatment tool at a slow response, and in a case where the stretching of the coupling member is large, the endoscope interlock with the forward and backward movement of a treatment tool at a quick response.