An overtube comprising: an overtube body having a distal end, a proximal end, and a longitudinal axis; a first distal end opening and a second distal end opening provided at a distal end of the overtube body; a first proximal end opening and a second proximal end opening provided at a proximal end of the overtube body; a first insertion passage that is provided along the longitudinal axis of the overtube body, allows the first distal end opening and the first proximal end opening to communicate with each other, and allows a first insertion part of a first medical instrument to be inserted therethrough so as to be movable forward and backward; a second insertion passage that is provided along the longitudinal axis of the overtube body, allows the second distal end opening and the second proximal end opening to communicate with each other, and allows a second insertion part of a second medical instrument to be inserted therethrough so as to be movable forward and backward; and a coupling mechanism that has a first coupling part coupled to the first insertion part inserted through the first insertion passage and a second coupling part coupled to the second insertion part inserted through the second insertion passage, characterized in that the coupling mechanism includes a partition wall member that is housed inside the overtube body and extends along the longitudinal axis and that has a partition wall between the first insertion passage and the second insertion passage, a first fixing tool that has the first coupling part and is movable forward and backward along the first insertion passage, a second fixing tool that has the second coupling part and is movable forward and backward along the second insertion passage, and a ring-shaped driving member that is externally fitted to an outer peripheral part of the partition wall member and is movable forward and backward along the longitudinal axis with respect to the partition wall member and that has a sensing region where either the first fixing tool or the second fixing tool is moved forward and backward in an interlocking manner with the forward and backward movement of the other of the first fixing tool and the second fixing tool. an overtube body having a distal end, a proximal end, and a longitudinal axis; a first distal end opening and a second distal end opening provided at a distal end of the overtube body; a first proximal end opening and a second proximal end opening provided at a proximal end of the overtube body; a first insertion passage that is provided along the longitudinal axis of the overtube body, allows the first distal end opening and the first proximal end opening to communicate with each other, and allows a first insertion part of a first medical instrument to be inserted therethrough so as to be movable forward and backward; a second insertion passage that is provided along the longitudinal axis of the overtube body, allows the second distal end opening and the second proximal end opening to communicate with each other, and allows a second insertion part of a second medical instrument to be inserted therethrough so as to be movable forward and backward; and a coupling mechanism that has a first coupling part coupled to the first insertion part inserted through the first insertion passage and a second coupling part coupled to the second insertion part inserted through the second insertion passage, characterized in that the coupling mechanism includes a partition wall member that is housed inside the overtube body and extends along the longitudinal axis and that has a partition wall between the first insertion passage and the second insertion passage, a first fixing tool that has the first coupling part and is movable forward and backward along the first insertion passage, a second fixing tool that has the second coupling part and is movable forward and backward along the second insertion passage, and a ring-shaped driving member that is externally fitted to an outer peripheral part of the partition wall member and is movable forward and backward along the longitudinal axis with respect to the partition wall member and that has a sensing region where either the first fixing tool or the second fixing tool is moved forward and backward in an interlocking manner with the forward and backward movement of the other of the first fixing tool and the second fixing tool. a partition wall member that is housed inside the overtube body and extends along the longitudinal axis and that has a partition wall between the first insertion passage and the second insertion passage, a first fixing tool that has the first coupling part and is movable forward and backward along the first insertion passage, a second fixing tool that has the second coupling part and is movable forward and backward along the second insertion passage, and a ring-shaped driving member that is externally fitted to an outer peripheral part of the partition wall member and is movable forward and backward along the longitudinal axis with respect to the partition wall member and that has a sensing region where either the first fixing tool or the second fixing tool is moved forward and backward in an interlocking manner with the forward and backward movement of the other of the first fixing tool and the second fixing tool.