An overtube that guides an endoscope including an endoscope insertion part having an observation part provided at a distal end thereof, and a treatment tool including a treatment tool insertion part having a treatment part provided at a distal end thereof into a body cavity, the overtube comprising: an overtube body that passes through a body wall and is inserted into the body cavity; an endoscope insertion passage that is provided inside the overtube body and allows the endoscope insertion part to be inserted therethrough so as to be movable forward and backward; a treatment tool insertion passage that is provided inside the overtube body and allows the treatment tool insertion part to be inserted therethrough so as to be movable forward and backward; and an interlocking member that includes an endoscope engagement part (420, 540) engaged with the endoscope insertion part inserted through the endoscope insertion passage, and a treatment tool engagement part (422, 542) engaged with the treatment tool insertion part inserted through the treatment tool insertion passage and the interlocking member being arranged inside the overtube body so as to be slidable forward and backward in an axial direction of the overtube body, wherein an axial direction of the endoscope insertion passage is an oblique direction with respect to an axial direction of the treatment tool insertion passage, and is provided in a direction in which the endoscope insertion part and the treatment tool insertion part are provided in directions away from each other at a distal end side, and the interlocking member includes an adjusting mechanism that adjusts a distance in a direction perpendicular to the axial direction of the overtube body between the endoscope engagement part (420, 540) and the treatment tool engagement part (422, 542) according to a position of the interlocking member in the axial direction of the overtube body an overtube body that passes through a body wall and is inserted into the body cavity; an endoscope insertion passage that is provided inside the overtube body and allows the endoscope insertion part to be inserted therethrough so as to be movable forward and backward; a treatment tool insertion passage that is provided inside the overtube body and allows the treatment tool insertion part to be inserted therethrough so as to be movable forward and backward; and an interlocking member that includes an endoscope engagement part (420, 540) engaged with the endoscope insertion part inserted through the endoscope insertion passage, and a treatment tool engagement part (422, 542) engaged with the treatment tool insertion part inserted through the treatment tool insertion passage and the interlocking member being arranged inside the overtube body so as to be slidable forward and backward in an axial direction of the overtube body, wherein an axial direction of the endoscope insertion passage is an oblique direction with respect to an axial direction of the treatment tool insertion passage, and is provided in a direction in which the endoscope insertion part and the treatment tool insertion part are provided in directions away from each other at a distal end side, and the interlocking member includes an adjusting mechanism that adjusts a distance in a direction perpendicular to the axial direction of the overtube body between the endoscope engagement part (420, 540) and the treatment tool engagement part (422, 542) according to a position of the interlocking member in the axial direction of the overtube body an endoscope insertion passage that is provided inside the overtube body and allows the endoscope insertion part to be inserted therethrough so as to be movable forward and backward; a treatment tool insertion passage that is provided inside the overtube body and allows the treatment tool insertion part to be inserted therethrough so as to be movable forward and backward; and an interlocking member that includes an endoscope engagement part (420, 540) engaged with the endoscope insertion part inserted through the endoscope insertion passage, and a treatment tool engagement part (422, 542) engaged with the treatment tool insertion part inserted through the treatment tool insertion passage and the interlocking member being arranged inside the overtube body so as to be slidable forward and backward in an axial direction of the overtube body,