The overtube includes an interlocking member which is configured to be movable forward and backward inside an overtube body—and has an endoscope-coupled part and a treatment tool-coupled part. A side hole that is formed to pass through a side wall of the overtube body in communication with a passage in the overtube body for sending a fluid into a body cavity, and a side wall is configured to be openable and closable by the forward and backward movement of the interlocking member.